The correct answer is option 2: "Help in nitrogen economy of the plant."

Legumes, which include plants like peas, beans, and lentils, have a symbiotic relationship with specific bacteria called rhizobia. These bacteria live in nodules found on the roots of legume plants. This relationship allows the legumes to convert atmospheric nitrogen into a form that plants can use, known as nitrogen fixation.

Nitrogen is an essential nutrient for plant growth, and it is often a limiting factor in agriculture. By hosting rhizobia and engaging in nitrogen fixation, legumes help to increase the availability of nitrogen in the soil. This benefits not only the legume plants themselves but also other crops grown in rotation with legumes.

Option 1, "Need very little water to survive," is not a key reason why legumes are important in agriculture. While it is true that some legume species are relatively drought-tolerant, their ability to fix nitrogen is the main reason for their significance.

Option 3, "Are disease resistant," is not a primary characteristic of legumes in general. While some legume varieties may have disease resistance traits, it is not a universal feature of all legumes.

Option 4, "Have a short life," is not accurate