Exp) Option a is the correct answer.

Nitrogen fixation is a process that implies the transformation of the relatively non-reactive atmospheric N2 into its more reactive compounds (nitrates, nitrites, or ammonia).

Nitrogen-Fixing Plants: An N-fixing crop is a natural way to provide plant-adjusted N without any industrial harm to nature. Nitrogen fixing plants are:

• Clovers, vetches, and peas are nitrogen-fixing plants used by farmers worldwide and in the US Southern Great Plains in particular.
• Peas or beans can be used as a summer nitrogen-fixing cover crop or harvested for food. Both ways, they enrich the soil with plant-suitable N. Southern peas prefer warm seasons and are sown when the soil temperature reaches 60F.
• Beans: fava (aka faba, broad), alfalfa, green (aka French), runner, field, sweet, peanuts (aka groundnuts), soybeans, cream, black-eyed, or purple-hulled beans, lupins, lentils, cowpeas,

Source:  

https://eos.com/blog/nitrogen-fixation/

https://www.britannica.com/plant/chickpea