Step 1: Read the question.
We must find the microbe that makes root nodules in Alnus, a non-legume tree.
Step 2: Recall nodule formation.
Root nodules are little swellings on roots where bacteria live and fix nitrogen from the air for the plant.
Step 3: Legume versus non-legume.
In legumes like pea and bean, the partner is Rhizobium. But Alnus is not a legume, so the partner is different.
Step 4: The correct partner.
In Alnus the nitrogen-fixing nodules are formed by Frankia, a filamentous bacterium that teams up with many non-leguminous plants.
Step 5: Rule out the rest.
Rhizobium works only with legumes. Nostoc is a cyanobacterium that fixes nitrogen but pairs with Cycas and Anthoceros, not Alnus. Rhodospirillum is a free-living form and does not make nodules.
Step 6: Conclusion.
Therefore the correct answer is "Frankia". \[ \boxed{\text{Frankia}} \]