Explanation:

- Natural silk is a natural protein fiber, primarily composed of fibroin.

- Fibroin is a protein made up of several amino acids.

- Nitrogen is a key element in amino acids, making it a constituent of silk.

- Hence, Nitrogen is indeed a constituent of natural silk.

- Potassium and magnesium are minerals essential for various biological processes, not directly involved in silk composition.

- Phosphorus is vital for energy storage and transfer in living cells but not directly linked to silk structure.

Correct Answer: Option 3 - Nitrogen