Mastication refers to the process of chewing food in the mouth. It is the first step of digestion, where food is broken down into smaller pieces by the teeth and mixed with saliva. Saliva contains enzymes such as amylase, which begin the chemical digestion of carbohydrates. Chewing also increases the surface area of food, making it easier for digestive enzymes to act. Assimilation refers to the use of absorbed nutrients by the body, absorption is the uptake of nutrients into the bloodstream, and excretion is the removal of waste. Thus, chewing of food is the correct answer. Mastication is essential for proper digestion and nutrient absorption. Understanding digestive processes is important in biology and health sciences.

Sulphur dioxide causes corrosion by forming sulphuric acid in presence of moisture, damaging iron and steel structures over time. It accelerates rusting and weakens metal integrity, especially in polluted industrial environments.

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.

The Rio Earth Summit was held in June 1992 in Brazil and focused on sustainable development and environmental protection.

Producers, mainly green plants, require nitrogen for synthesizing proteins, nucleic acids, and chlorophyll. However, they cannot directly use atmospheric nitrogen (N₂), which is chemically inert. Instead, they absorb nitrogen in usable forms such as ammonia (NH₃), ammonium ions (NH₄⁺), or nitrates (NO₃⁻). Ammonia is produced through biological nitrogen fixation by certain bacteria, which convert atmospheric nitrogen into a form plants can absorb. This process is crucial for maintaining the nitrogen cycle in ecosystems. CO₂ is used by plants in photosynthesis but does not provide nitrogen. N₂, although abundant in the atmosphere, cannot be directly utilized by plants without conversion. NH₄NO₃ is a fertilizer that provides nitrogen, but the fundamental absorbed form at the biochemical level is ammonia/ammonium. Thus, NH₃ is the correct answer. Understanding nutrient cycles is essential for grasping ecosystem functioning and is a common topic in competitive exams. This concept highlights the importance of microorganisms in supporting plant life and maintaining ecological balance.