Plants take nitrogen in the form of

Answer: (c) Nitrates

Plants mainly absorb nitrogen as nitrate (NO₃⁻) through their roots.

The idea

Plants cannot use nitrogen gas. They take nitrogen from soil water through the roots, and the form that dissolves and moves most easily in soil water is nitrate. Soil bacteria prepare it for them.

N₂ (air) → fixation → NH₃ → nitrite (NO₂⁻) → nitrate (NO₃⁻) → roots

Why the others are wrong

(a) Nitrite is a toxic in-between step (Nitrosomonas makes it, Nitrobacter turns it into nitrate).

(b) Some ammonium is taken up, but the main form absorbed by most plants is nitrate.

(d) N₂ gas has a triple bond that plants cannot break — only fixing bacteria can.

The trap

78% of air is nitrogen, so “element” tempts. Plants can’t use N₂ at all.

Also know

The whole cycle in five steps: fixation (N₂ to ammonia), nitrification (ammonia to nitrite to nitrate), assimilation (plants take up nitrate), ammonification (dead matter back to ammonia) and denitrification (nitrate back to N₂, in waterlogged soil).