A common phycobiont in lichens are

The correct answer is Trebouxia.

Understanding Lichens:
A lichen is a composite organism that arises from a mutually beneficial symbiotic relationship between two main partners:
1. Phycobiont (or Photobiont): The photosynthetic partner, which is typically a green alga or a cyanobacterium. It is responsible for synthesizing organic food through photosynthesis.
2. Mycobiont: The fungal partner, which provides structural support, protection, and absorbs water and minerals from the environment.

Role and Identification of Trebouxia:
Among the photosynthetic partners (phycobionts), unicellular green algae are the most common. In particular, the genus Trebouxia is a unicellular green alga that serves as the most widespread and common phycobiont, association-wise, in more than 50% to 70% of all known lichen species (especially macrolichens). Other genera like Oedogonium are free-living filamentous green algae, Cetraria is itself a genus of lichen (a mycobiont dominant structure), and Microcystis is a genus of planktonic cyanobacteria not typically forming lichen associations. Therefore, Trebouxia is the correct phycobiont partner.