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.

Karakoram Highway length is about 1600 km connecting Pakistan and China.

Edward Jenner, an English physician, is credited with developing the first successful vaccine in 1796. He discovered that milkmaids who had contracted cowpox were immune to smallpox, a deadly disease at the time. Jenner tested his theory by inoculating a young boy with material from cowpox lesions and later exposing him to smallpox. The boy did not develop the disease, proving immunity. This discovery laid the foundation for immunology and modern vaccination. It led to the eventual eradication of smallpox worldwide. Other options are incorrect as they do not match the historical timeline. Thus, 1796 is the correct answer, marking a major breakthrough in medical science and public health.

Zahid Hussain was the first Governor of the State Bank of Pakistan when it was established in 1948. The State Bank serves as the central bank of the country and is responsible for monetary policy, currency issuance, and financial stability. As the first governor, Zahid Hussain played a crucial role in setting up the banking system of the newly independent Pakistan. His leadership helped establish financial institutions and regulatory frameworks. Other options include later officials or unrelated figures. Shamshad Akhtar served much later, while Malik Ghulam Muhammad was a political figure. Thus, Zahid Hussain is the correct answer, marking an important milestone in Pakistan’s economic history.

A parsec is a unit of distance used in astronomy. It is defined as the distance at which one astronomical unit subtends an angle of one arcsecond. One parsec is approximately equal to 3.26 light-years. It is commonly used to measure distances between stars and galaxies because these distances are extremely large. Speed, time, and acceleration are different physical quantities and do not apply here. Therefore, distance is the correct answer. Parsec is especially useful in parallax measurements, where astronomers calculate distances based on the apparent shift of a star’s position.