Proteins are called body-building nutrients because they help in growth and repair of body tissues.
Step 1: Analyze the options.
- Carbohydrates — Energy giving
- Proteins — Body building ✔
- Fats — Energy storage
- Vitamins — Protective nutrients
Step 2: Conclusion.
Thus proteins are responsible for body building.
Final Answer:
\[
\boxed{(B) \, \text{Proteins}}
\]