Azolla is a type of water fern that has a symbiotic relationship with a cyanobacterium called Anabaena. This partnership allows Azolla to fix atmospheric nitrogen, converting it into a form that can be utilized by plants. This nitrogen fixation ability makes Azolla a valuable biofertilizer for rice cultivation.

Option 1: Maize cultivation - There is no direct evidence to suggest that Azolla significantly benefits maize cultivation.

Option 2: Wheat cultivation - Azolla is not commonly used as a biofertilizer for wheat cultivation.

Option 3: Barley cultivation - Like maize and wheat, Azolla is not commonly used for barley cultivation.

Option 4: Rice cultivation - Azolla is known to enhance soil fertility and increase nitrogen availability for rice plants. It forms a dense mat on the water surface, resulting in reduced weed growth, increased organic matter, and improved soil structure. Therefore, Azolla is most commonly used for rice cultivation.

Therefore, the correct answer is option 4, rice cultivation.