Step 1: Concept Overview:
The problem requires identifying the plant exhibiting symbiosis with the nitrogen-fixing cyanobacterium Anabaena.
Step 2: Detailed Analysis:
The symbiotic relationship between the aquatic fern {Azolla} and the cyanobacterium {Anabaena azollae} exemplifies mutualism.
Anabaena resides within leaf cavities of Azolla, not roots. Although "root pockets" is imprecise (leaf cavities are correct), it alludes to the Azolla-Anabaena symbiosis. The cyanobacterium provides fixed nitrogen to the fern. The fern offers protection and carbohydrates to Anabaena.
Pistia, Marsilea, and Salvinia, while aquatic, lack this specific symbiotic relationship with Anabaena.
Step 3: Conclusion:
Despite the term "root pockets" being technically incorrect (leaf cavities are intended), Azolla is the correct answer because of its established symbiosis with Anabaena.