Algae which form motile colony is
The algae that forms motile colonies is Volvox. Volvox is the most developed in a series of genera, that form spherical colonies.
Each mature Volvox colony is composed of numerous flagellate cells similar to Chlamydomonas, up to 50,000 in total, and embedded in the surface of a hollow sphere or coenobium containing an extracellular matrix made of a gelatinous glycoprotein. Except, during the formation of daughter colonies, vegetative cells comprise a single layer with the flagella facing outward. The cells swim in a coordinated fashion, with distinct anterior and posterior poles. Hence, form a motile colonies. Thus, the correct answer is option A.