## **Core Concept**
Nitrogen fixation is a process by which atmospheric nitrogen (N₂) is converted into a form that can be used by living organisms, such as ammonia (NH₃) or nitrate (NO₃⁻). This process is crucial because nitrogen is an essential nutrient for plant growth, but most plants cannot use atmospheric nitrogen directly. Certain plants, known as legumes, have symbiotic relationships with nitrogen-fixing bacteria.

## **Why the Correct Answer is Right**
The correct answer, **C. Legume**, is right because legumes have nodules on their roots where Rhizobium bacteria reside. These bacteria convert atmospheric nitrogen into a form that the plant can use, such as ammonium. This symbiotic relationship benefits both the plant, which receives a usable form of nitrogen, and the bacteria, which receive carbohydrates produced by the plant during photosynthesis. Examples of legumes include peas, beans, and lentils.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it is not specified, but generally, plants like those in options A and B do not have the specialized root nodules necessary for nitrogen fixation.
- **Option B:** This option is incorrect for similar reasons as option A; without specifying the type of plant, it's hard to directly refute, but typically, non-legume plants do not fix nitrogen.
- **Option D:** This option is incorrect because, like options A and B, it does not accurately represent a category of plants known for nitrogen fixation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that legumes, due to their ability to fix nitrogen through symbiotic bacteria, often do not require additional nitrogen fertilizers, making them beneficial in crop rotation for maintaining soil fertility.

## **Correct Answer:** C. Legume.