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.

The human brain is composed of approximately 75% water, making it one of the most water-rich organs in the body. Water is essential for maintaining brain function, including nutrient transport, temperature regulation, and electrical activity. Adequate hydration is crucial for cognitive functions such as memory, concentration, and alertness. Even mild dehydration can impair brain performance. The other percentages are either too low or too high to accurately represent the brain’s composition. Thus, 75% is the correct answer. This emphasizes the importance of water in maintaining overall health and brain function.

Pepsinogen is an inactive precursor, or zymogen, secreted by the gastric glands in the stomach lining. It is converted into its active form, pepsin, in the presence of hydrochloric acid (HCl). This acidic environment triggers the activation process. Pepsin is a proteolytic enzyme that begins the digestion of proteins by breaking them down into smaller peptides. This process is essential for further digestion and absorption in the small intestine. The conversion of pepsinogen into pepsin is a protective mechanism. If pepsin were active inside the cells that produce it, it could digest the stomach lining itself. By secreting it in an inactive form, the body prevents self-damage. Other options are incorrect: “enzyme” is too general, proteins are the substrate, and amylase digests carbohydrates. Thus, pepsin is the correct answer. This concept is important for understanding enzyme activation and digestive physiology, commonly tested in exams.

Pakistan’s population constitutes approximately 2.65% of the world’s total population according to recent UN estimates. This makes Pakistan one of the most populous countries globally. Population statistics are important for planning economic development, resource allocation, and policy-making. A large population also presents both opportunities and challenges, including workforce potential and pressure on resources. Thus, 2.65% is the correct answer based on recent demographic data.

Kurt Waldheim served as the fourth Secretary-General of the United Nations from 1972 to 1981. The Secretary-General is the chief administrative officer of the UN and plays a key role in international diplomacy. Waldheim’s tenure focused on global peace efforts and conflict resolution during the Cold War period. His role involved mediating disputes and representing the UN on international platforms. Thus, he is ranked as the 4th Secretary-General, making option D correct.