Blue-green algae are chosen as the correct answer as they have been widely used as a biofertilizer for rice crops. These algae have the ability to fix atmospheric nitrogen, which enriches soil quality and promotes rice growth by providing necessary nutrients.

Option 2, Rhizobium sp, are nitrogen-fixing bacteria but usually form a symbiotic relationship with leguminous plants such as peas, beans, etc. Hence, they won`t directly benefit rice plants.

Option 3, Mycorrhizal fungi, form a mutualistic relationship with the roots of most types of plants and help in nutrient uptake. However, their effect on rice plants isn`t as significant as blue-green algae.

Option 4, Azotobacter sp, are free-living nitrogen-fixing bacteria present in soil. They contribute to soil fertility but aren`t as specific for rice crop as blue-green algae.

So, only Blue-green algae among the mentioned options are directly and significantly beneficial for rice crops.