Symbiotic nitrogen fixation — Choose the classic example of a symbiotic nitrogen-fixing bacterium forming root nodules on legumes.

Introduction / Context:
Symbiotic nitrogen fixation is a cornerstone of sustainable agriculture. Certain bacteria infect legume roots and form nodules where nitrogenase reduces N2 to ammonia, supplying nitrogen directly to the host plant in exchange for carbon compounds and a protected, low-oxygen niche.

Given Data / Assumptions:

• We distinguish symbiotic nodule-forming bacteria from free-living diazotrophs.
• Legume symbioses are the best-known agricultural examples.

Concept / Approach:
Rhizobium (including Bradyrhizobium and related genera) forms nodules on legumes (e.g., peas, beans, soy). By contrast, Azotobacter, Beijerinckia, and Clostridium are free-living nonsymbiotic fixers and do not form legume nodules.

Step-by-Step Solution:

Verification / Alternative check:
Inoculant packets for legumes are labeled with specific Rhizobium/Bradyrhizobium strains tailored to host species, underscoring their symbiotic nature.

Why Other Options Are Wrong:

• Azotobacter, Beijerinckia, Clostridium: nonsymbiotic; do not form legume nodules.

Common Pitfalls:
Assuming any nitrogen fixer is symbiotic; many are free-living and do not create nodules.

Final Answer:
Rhizobium