Step 1: Problem Definition:
The question requires identifying plants exhibiting symbiotic relationships with nitrogen-fixing microorganisms (bacteria or cyanobacteria).
Step 2: Detailed Analysis:
Analyzing each plant:
A. Azolla: This aquatic fern forms a symbiosis with the cyanobacterium Anabaena azollae, located in leaf cavities. This symbiotic relationship makes Azolla an effective biofertilizer, especially in rice cultivation. (Correct)
B. Gnetum: This gymnosperm lacks symbiotic nitrogen-fixing relationships. (Incorrect)
C. Anthoceros: This hornwort possesses thallus cavities colonized by the nitrogen-fixing cyanobacterium Nostoc. (Correct)
D. Cycas: This gymnosperm features specialized 'coralloid roots' that host symbiotic nitrogen-fixing cyanobacteria like Nostoc or Anabaena. (Correct)
E. Riccia: This liverwort does not engage in symbiotic nitrogen fixation. (Incorrect)
Step 3: Conclusion:
Azolla, Anthoceros, and Cycas establish symbiotic relationships with nitrogen-fixing bacteria. Therefore, the answer is A, C, and D only.