Concept:A colony is a group of similar cells living together in association, and Volvox is a well-known colonial organism.
Explanation:Volvox forms hollow, spherical colonies made up of many cells embedded in a gelatinous matrix.
The cells cooperate and remain connected, which is the key feature of a colonial organism.
Paramecium and Euglena are single-celled organisms.
Chlamydomonas is also a unicellular alga, though structurally similar to individual Volvox cells.
Therefore, the organism that exists as a colony is Volvox.
Answer:A. Volvox