The correct answer is option 2, Pea. The roots of pea plants contain a type of bacteria called Rhizobia. These bacteria have a symbiotic relationship with pea plants. The bacteria colonize the roots of pea plants and form nodules. Inside these nodules, the bacteria convert free nitrogen from the air into a form that the plants can use, called nitrates. This process is called nitrogen fixation.

This conversion of nitrogen gas into nitrates is crucial for plant growth, as nitrogen is an essential nutrient for plants. Nitrates serve as a source of nitrogen for the plants, which they need for the production of proteins and other important molecules.

Although other plants, such as legumes, can also form symbiotic relationships with nitrogen-fixing bacteria, the roots of pea plants specifically contain the bacteria that convert free nitrogen to nitrates. Mustard, sugarcane, and wheat are not known for having this specific symbiotic relationship with nitrogen-fixing bacteria in their root systems.

In summary, the roots of pea plants contain bacteria called Rhizobia, which convert free nitrogen to nitrates that can be used by the plants.