The common phycobiont in a lichen isa. Microcys

The common phycobiont in a lichen is Trebouxia.

Explanation:

A lichen is a symbiotic association between a fungus and a photosynthetic partner, which can be either an alga or a cyanobacterium. The photosynthetic partner is known as the "phycobiont."
Trebouxia is a genus of green algae that is commonly found as the phycobiont in lichen symbiosis. It is one of the most widespread and common algal partners in lichens.
The other options listed (Microcystis, Ulothrix, and Spirogyra) are different types of algae or cyanobacteria, but they are not typically found as the phycobiont in lichens.
Thus, the correct answer is b. Trebouxia.