Correct Answer: A

Rationale: mRNA translation typically occurs on the ribosome. Ribosomes are the cellular machinery responsible for protein synthesis. They read the mRNA sequence and assemble amino acids into a polypeptide chain during translation. The correct answer is A. Choices B, C, and D are incorrect. The nucleus is where mRNA is transcribed from DNA, while translation occurs in the cytoplasm on ribosomes. The smooth endoplasmic reticulum is involved in protein processing and transport but not in mRNA translation.

Correct Answer: B

Rationale: The correct answer is B: 'To fight infection.' The immune system's primary function is to protect the body from infections and foreign invaders by recognizing and responding to pathogens. It plays a crucial role in maintaining overall health and preventing diseases caused by harmful microorganisms. Choices A, C, and D are incorrect because the immune system is not responsible for transporting oxygen, digesting food, or producing hormones. These functions are carried out by other systems in the body, such as the respiratory system, digestive system, and endocrine system, respectively.

Correct Answer: B

Rationale: Responsiveness is the correct term that refers to the ability to detect and respond to changes in the internal and external environment. In this context, growth is incorrect as it pertains to an increase in size or number, reproduction involves producing offspring, and development refers to the process of growth, maturation, and differentiation of cells and tissues. Therefore, 'Responsiveness' is the most appropriate term for the given description.

Correct Answer: B

Rationale: The correct answer is B, Skeletal muscle. Skeletal muscle is both voluntary and striated, as well as attached to bones, allowing for movement. Smooth muscle, on the other hand, is involuntary and typically found in the walls of internal organs, controlling functions like digestion. Cardiac muscle is also striated, but it is involuntary and specifically found in the heart, responsible for cardiac contractions to pump blood. 'Epithelial muscle' is an incorrect term; epithelial tissue is a type of tissue that forms the linings of various body surfaces and structures, not a muscle type.

Correct Answer: A

Rationale: The correct answer is A: B cells, a type of white blood cell, are responsible for producing antibodies in response to pathogens. Antibodies play a crucial role in identifying and neutralizing foreign substances like bacteria and viruses in the body, aiding in the immune response and protecting against infections. This function is essential for maintaining a healthy immune system. White blood cells, including B cells, are produced in the bone marrow before entering the bloodstream. Platelets, not white blood cells, are a specialized class of blood cells that function to clot blood and stop bleeding. While white blood cells provide immunity, not all of them are active throughout life as some have specific lifespans and functions.