Which one of the following plants can fix nitrogen from air?

Answer: (c) Pea

Pea is a legume: Rhizobium bacteria in its root nodules fix nitrogen from the air.

The idea

Strictly, no plant fixes nitrogen. Bacteria do. The two nitrogen atoms in N₂ are joined by a very strong triple bond, and only certain bacteria have the enzyme (nitrogenase) to break it. A legume gives these bacteria a home in its root nodules.

Rhizobium in the soil enters the root hair → The root forms a nodule around it → Inside, bacteria turn N₂ into ammonia → The plant uses the ammonia → The plant gives sugar in return

Why the others are wrong

(a) Rice fields gain nitrogen from blue-green algae (Anabaena in Azolla) in the paddy water, but the rice plant itself does not fix it.

The trap

Rice fields do gain fixed nitrogen — from Azolla and cyanobacteria, not from the rice plant.

Also know

A healthy nodule is pink inside because of leghaemoglobin, which keeps oxygen away from the enzyme. In rice fields the small water fern Azolla, with the blue-green alga Anabaena inside its leaves, does the same job; it is used as a biofertiliser.