Which of the angiosperm contains Nostoc as an endophyte?

The correct answer is Gunnera manicata.

Step-by-step Explanation:
1. Understanding Endophytes: An endophyte is an organism (often a bacterium or fungus) that lives inside a plant partner in a symbiotic relationship, typically without causing any disease.
2. Nostoc as a Symbiont: Nostoc is a genus of blue-green algae (cyanobacteria) capable of fixing atmospheric nitrogen. It forms symbiotic associations with various plants, including bryophytes (like hornworts), pteridophytes (like the water fern Azolla), gymnosperms (like the coralloid roots of Cycas), and certain angiosperms.
3. Association with Angiosperms: Among flowering plants (angiosperms), the genus Gunnera is uniquely known for its symbiotic association with Nostoc.
4. Mechanism in Gunnera: In the species Gunnera manicata, the cyanobacterium Nostoc enters specialized glands located at the base of the petioles on the stem. Once inside the host cells, the intracellular Nostoc provides fixed nitrogen to the plant, while the plant provides carbohydrates and shelter to the cyanobacterium.
5. Evaluating Other Options:
- Arachis hypogea (peanut plant) associates with nitrogen-fixing Rhizobium bacteria in root nodules, not Nostoc.
- Melia indica (Neem, often classified under Azadirachta indica) does not form endosymbiotic relationships with Nostoc.