Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are primarily composed of proteins, which are made up of amino acids folded into specific three-dimensional structures. This structure is crucial because it determines the enzyme’s active site, where substrates bind and reactions occur. Enzymes are highly specific, meaning each enzyme typically catalyzes a particular reaction. Vitamins and minerals may act as cofactors or coenzymes, assisting enzyme function, but they are not the main structural components. Carbohydrates do not form enzymes. Thus, proteins are the correct answer. This concept is fundamental in biochemistry and helps explain how metabolic processes occur efficiently in living organisms.

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.

Plants absorb nitrogen in usable forms like ammonia (NH3) or nitrates. Atmospheric nitrogen cannot be directly used without fixation.

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.

Rainfall is measured using a rain gauge, a meteorological instrument designed to collect and measure precipitation. It typically consists of a funnel that directs rainwater into a measuring cylinder. The amount of rainfall is recorded in millimeters, representing the depth of water that would accumulate on a flat surface. Accurate rainfall data is essential for agriculture, flood prediction, and weather forecasting. A hydrometer measures liquid density, not rainfall. “Rain meter” and “hydroscope” are not standard instruments. Thus, rain gauge is the correct answer. It is widely used by meteorological departments worldwide.