Step 1: Understanding the Question.
The question asks which nutrient is mainly responsible for building and repairing body tissue, such as muscle, skin and internal organs.
Step 2: Key Concept.
Body tissues are made of cells, and cells are largely built from protein. Protein supplies the amino acids needed to grow new tissue, repair damaged cells and replace old worn out cells, which is why it is often called the body building nutrient.
Step 3: Checking each option.
(A) Protein: Protein breaks down into amino acids, which the body then uses to build new muscle, skin and other tissue, making it the true tissue builder.
(B) Fat: Fat mainly stores energy and cushions organs, it plays only a small role in tissue building compared to protein.
(C) Carbohydrate: Carbohydrate is chiefly used to supply quick energy, not to build tissue.
(D) Vitamin: Vitamins support many body functions and help enzymes work well, but they do not themselves form the structure of tissue.
Step 4: Final Answer.
Protein is the nutrient that builds tissues in the body.