Stomata are tiny openings on the surface of leaves that regulate gas exchange and water loss. They are controlled by guard cells, which change shape based on water content. When guard cells gain water, they become turgid and open the stomata, allowing carbon dioxide to enter for photosynthesis. When they lose water, they become flaccid and close the stomata to prevent excessive water loss through transpiration. This mechanism is essential for plant survival, especially in dry conditions. It helps maintain water balance while still allowing necessary gas exchange. The other options are incorrect because gaining water opens stomata, and mutation is unrelated. Thus, stomata close when guard cells lose water, making option A correct. This concept is fundamental in plant physiology and ecology.

Benjamin Franklin, a prominent American scientist and statesman, invented both the lightning rod and bifocal spectacles. His lightning rod invention emerged from his experiments with electricity, including the famous kite experiment, which demonstrated that lightning is a form of electrical discharge. The lightning rod protects buildings by directing electrical energy safely into the ground, preventing fires and structural damage. This invention had immediate practical importance and is still widely used today. Franklin also invented bifocal spectacles to address his own vision problems. These glasses combine two lenses in one frame, allowing a person to see both near and distant objects clearly without switching glasses. This was a major convenience and innovation in optics. Other options are notable scientists but unrelated to both inventions: Thomas Edison developed electric devices, Alessandro Volta invented the battery, and Benjamin Banneker was a mathematician and astronomer. Therefore, Benjamin Franklin is the correct answer, as he contributed significantly to both electrical science and practical inventions improving daily life.

The term “bit” is short for “binary digit,” which is the smallest unit of data in computing and digital communications. A bit can have only two possible values: 0 or 1. These values represent the fundamental building blocks of all digital systems, including computers, smartphones, and communication networks. Bits are used to represent data in binary form because electronic systems operate using two states, such as on/off or high/low voltage. Multiple bits are combined to form larger data units such as bytes (8 bits), kilobytes, megabytes, and so on. Understanding bits is essential for grasping how computers process, store, and transmit information. Higher-level data like text, images, and videos are all ultimately encoded in binary form using bits. Other options are incorrect because they do not represent the standard definition of “bit.” Thus, binary digit is the correct answer, forming the foundation of all modern computing systems.

The Uch Gas Field is located in Balochistan, Pakistan, and is an important source of natural gas for the country. It was discovered in the mid-20th century and has been developed primarily to supply fuel for power generation. Natural gas from Uch is used in thermal power plants, helping to generate electricity and support industrial activities. This contributes significantly to Pakistan’s energy sector and reduces reliance on imported fuels. Balochistan is rich in natural resources, including gas, coal, and minerals. Other provinces like Sindh also have major gas fields, such as Qadirpur, but Uch specifically belongs to Balochistan. Khyber Pakhtunkhwa and Punjab are not known for the Uch gas field. Therefore, Balochistan is the correct answer. Knowledge of such resources is important for understanding national energy infrastructure and economic geography.

The Rio Earth Summit, formally called the United Nations Conference on Environment and Development (UNCED), was held in June 1992 in Rio de Janeiro, Brazil. It was one of the largest international gatherings focused on environmental issues and sustainable development. The summit produced several key outcomes, including Agenda 21, a comprehensive plan for sustainable development; the Rio Declaration, which outlines principles for environmental protection; and major conventions on climate change and biodiversity. These agreements laid the foundation for modern environmental policy and global cooperation. The summit emphasized the concept of sustainable development, which balances economic growth with environmental protection. It highlighted the responsibility of nations to protect natural resources while promoting development. Other dates listed are incorrect because the conference specifically took place in June 1992. It remains a landmark event in environmental history. Thus, June 1992 is the correct answer, representing a pivotal moment in global environmental awareness and policy-making.