An organism that forms a symbiotic association with Anthoceros

The correct option is Nostoc.

Step-by-step Explanation:

1. Understanding Anthoceros:
Anthoceros is a genus of hornworts, which are non-vascular plants belonging to the division Bryophyta. The plant body (thallus) of Anthoceros is lobed and dorsiventrally flattened.

2. The Symbiotic Association:
Within the thallus of Anthoceros, there are specialized intercellular cavities filled with mucilage (mucilage cavities). These cavities open to the outside environment through slime pores. A cyanobacterium (blue-green alga) called Nostoc enters through these pores and colonizes the mucilage cavities, forming a stable symbiotic relationship.

3. Role of the Symbionts:
This is a mutualistic relationship beneficial to both organisms:
- Nostoc: It is capable of fixing atmospheric nitrogen into ammonia. It provides fixed nitrogen to the Anthoceros plant, which helps the host grow in nitrogen-poor soils.
- Anthoceros: In return, the host plant provides shelter, nutrients (carbohydrates), and a moist environment to the Nostoc colonies.

4. Analyzing Other Options:
- Spirogyra: A free-living, filamentous green alga known for its spiral chloroplasts. It does not form symbiotic associations with plants.
- Ulothrix: A genus of filamentous green algae found in fresh and marine water. It is also free-living and does not form symbiotic associations with hornworts.