Correct Answer: A

Rationale: The correct answer is A: Ribosome. Ribosomes are the organelles responsible for protein synthesis in the cell. They are the cellular machinery where translation, the process of assembling proteins from amino acids based on mRNA sequences, occurs. Ribosomes can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum. The nucleus (choice B) houses DNA but is not directly involved in protein synthesis. Mitochondria (choice C) are responsible for energy production through cellular respiration, not protein synthesis. The Golgi apparatus (choice D) is involved in modifying, sorting, and packaging proteins for secretion, not in the synthesis of proteins.

Correct Answer: B

Rationale: Cytokinesis refers to the division of the cytoplasm of a parental cell between two daughter cells during cell division. It ensures that each daughter cell receives its share of organelles and cytoplasmic components necessary for cellular functions. Choice A, division of the nucleus, is incorrect as this process is known as mitosis. Choice C, separation of chromosomes, is incorrect as it refers to the process of chromatid separation during mitosis. Choice D, formation of spindle fibers, is incorrect as spindle fibers are involved in mitosis to separate chromosomes, not in cytokinesis.

Correct Answer: A

Rationale: The correct answer is A: Spinal cord. The central nervous system comprises the brain and spinal cord. The spinal cord plays a crucial role in transmitting information between the brain and the body. Choices B, C, and D are not part of the central nervous system. Peripheral nerves belong to the peripheral nervous system, whereas the brainstem and cerebellum are components of the brain, distinct from the central nervous system.

Correct Answer: A

Rationale: The correct answer is A. Macrophages are not found abundantly in the blood; they are present in tissues and organs. Macrophages are tissue-resident immune cells that play a key role in immune responses and are derived from monocytes produced in the bone marrow. Choice B is correct as macrophages are larger than neutrophils. Choice C is correct as macrophages are long-lived cells. Choice D is correct as macrophages originate in the bone marrow as monocytes and differentiate into macrophages once they migrate to various tissues and organs.

Correct Answer: B

Rationale: The correct answer is B. The diaphragm plays a crucial role in respiration by contracting and expanding the lungs. When the diaphragm contracts, it moves downward, creating more space in the chest cavity and allowing the lungs to expand. This expansion leads to a decrease in pressure inside the lungs, causing air to rush in. When the diaphragm relaxes, it moves back up, decreasing the space in the chest cavity and causing the lungs to deflate. This action increases the pressure in the lungs, leading to air being pushed out. Therefore, the diaphragm's main function is to facilitate the inhalation and exhalation of air by contracting and expanding the lungs. Choices A, C, and D are incorrect because the diaphragm's primary function is not to regulate air pressure in the lungs, store oxygen, or break down carbon dioxide. Instead, its main purpose is to aid in the mechanical process of breathing.