Crotalaria juncea, known as sun or sun hemp, is a tropical Asian plant of the legume family (Fabaceae). It is generally considered to have originated in India. It is now widely grown throughout the tropics and subtropics as a source of green manure, fodder and lignified fiber obtained from its stem. These provide both organic matter and nitrogen to the soil. Sunn hemp is also being looked at as a possible bio-fuel. It is most commonly used as green manure in India. Sunflower, sesame and bougainvillea are not used as green manure. Thus, option 'B' is correct.

Sun hemp (Crotalaria Juncea) is the most commonly used plant as green manure in India. Green manure refers to plants that are grown specifically to improve the fertility and health of the soil. They are typically grown and then incorporated into the soil to add organic matter and nutrients.

There are several reasons why sun hemp is a popular choice as green manure in India:

1. Nitrogen fixation: Sun hemp is a leguminous plant, which means it has the ability to form a symbiotic relationship with nitrogen-fixing bacteria in its root nodules. These bacteria convert atmospheric nitrogen into a form that can be used by plants. This process helps to increase the nitrogen content in the soil, which is essential for plant growth.

2. Biomass production: Sun hemp is known for its rapid growth and high biomass production. It can reach a height of up to 2-3 meters within 60-90 days of sowing. This means that a large amount of organic matter can be added to the soil when sun hemp is incorporated.

3. Weed suppression: Sun hemp has allelopathic properties, which means it releases chemicals that inhibit the growth of weeds. This can help in suppressing weed competition, allowing the main crop to grow more effectively.

4. Soil improvement: Sun hemp has deep taproots that can penetrate the soil, helping to improve soil structure and drainage. The organic matter and nitrogen added by sun hemp also enhance soil fertility and microbial activity.

5. Drought tolerance: Sun hemp is relatively drought-tolerant and can withstand dry conditions. This makes it a suitable green manure crop, especially in regions with limited water availability.

In addition to sun hemp, other plants like sesbania, cowpea, and dhaincha are also used as green manure in India. However, sun hemp is preferred due to its fast growth, nitrogen-fixing ability, and weed suppression properties. It is an economically viable option for farmers looking to improve soil fertility and reduce the use of synthetic fertilizers.