Concept:
Body-building foods are rich in proteins, which are essential for growth and repair of body tissues.
Step 1: Identify the nutrient-rich food.
Among the given options,
• Fruits -- Rich in vitamins and minerals.
• Vegetables -- Rich in vitamins, minerals and fibre.
• Millets -- Mainly provide carbohydrates.
• Pulses -- Rich source of proteins. \checkmark
Step 2: Final conclusion.
Hence, the food substance useful for growth is
\[
\boxed{\text{Pulses}.}
\]