Science (086) — CBSE Class 10 board question

(b) Nitrogen

Nitrogen is the essential element taken up from the soil by plants to synthesize proteins.

Proteins contain nitrogen as a key element. Plants absorb nitrogen from the soil (as nitrates/nitrites) to build amino acids and proteins. The other options — phosphorus, iron, and magnesium — serve different functions and are not the primary element for protein synthesis. This is a straightforward factual question; just recall that proteins = nitrogen.