Option 1 -> Millets are cereal crops that do not have nitrogen-fixing capabilities.
Option 2 -> Pulses are leguminous crops with root nodules containing Rhizobium bacteria that fix atmospheric nitrogen, enriching the soil.
Option 3 -> Sugarcane is a cash crop without nitrogen-fixing properties.
Option 4 -> Rajma (kidney beans) is actually a specific type of pulse, but "Pulses" as a category is the broader and more complete answer.
Hence, Option 2 (Pulses) -> Pulses belong to the legume family and have a symbiotic relationship with nitrogen-fixing bacteria in their root nodules, which convert atmospheric nitrogen into usable forms, thereby naturally restoring and enriching soil fertility. This makes them excellent rotation crops. -> correct