Explanation:

Casuarina trees belong to a group of non-leguminous plants that are capable of forming a symbiotic relationship with nitrogen-fixing actinobacteria. Specifically, the nitrogen-fixing ability of Casuarina is due to its association with the actinomycete bacterium $Frankia$, which forms root nodules where atmospheric nitrogen ($N_2$) is converted into ammonia ($NH_3$). This process is known as actinorhizal symbiosis. Option A, Rhizobium, is the bacterium responsible for nitrogen fixation in legumes (e.g., peas, beans, groundnuts). Options B, Azotobacter, and D, Anabaena, are free-living nitrogen-fixing bacteria and cyanobacteria, respectively, but are not the symbiotic partners for Casuarina.