The correct answer is (b). Plants receive nitrogen in the form of nitrate. Nitrate is a form of nitrogen that is easily absorbed by plant roots. It is a major component of fertilizers and is also found in natural sources such as soil and water.
Nitric oxide (a) is a gas that is produced by the reaction of nitrogen and oxygen. It is not a form of nitrogen that is easily absorbed by plants.
Ammonia (c) is a gas that is produced by the breakdown of organic matter. It is a form of nitrogen that is easily absorbed by plants, but it is not as readily available as nitrate.
Nitride (d) is a solid that is formed when nitrogen combines with other elements, such as hydrogen or oxygen. It is not a form of nitrogen that is easily absorbed by plants.