Question

Coenobium occurs in
A) Spirogyra
B) Ulothrix
C) Volvox
D) Chlorella

Answer 1

Coenobium is a colony of cells mostly found in algae. It is a colony with a fixed number of cells, with little or no specialization. They occur in several groups of algae. The cells are often embedded in a mucilaginous matrix and may be motile or non-motile. There are several examples of coenobium like- Volvox , Scenedesmus, Pediastrum and Hydrodictyon.

Complete answer:
Option A Spirogyra - Spirogyra is a filamentous green alga named for its helical or spiral arrangement of the chloroplast that is characteristic of the genus. There are about $400$ species of Spirogyra in the world, and they are mostly found in freshwater settings. It is also known as water silk, mermaid’s tresses and blanket weed. They can reproduce both sexually and asexually. In vegetative reproduction, fragmentation takes place, and Spirogyra simply undergoes.
Option A is not correct.
Option B Ulothrix - Ulothrix is a non-branching filamentous green algae genus that can be found in both fresh and saltwater. Its cells are normally as broad as they are long and they live in low temperature in spring or winter season. They reproduce in a mode of normally vegetative.
Option B is not correct.
Option C Volvox - Volvox as we know, that it is a chlorophyte green alga in the family Volvocaceae. They live in a colonial cell in a variety of freshwater habitats. They live in pond water too, the color in the pond looks greenish due to the rapid growth of Volvox. Some examples- of Volvox like- Volvox aureus etc.
Option C is correct.
Option D Chlorella - Chlorella is a genus of about thirteen species of single-celled green algae. The cells are spherical in shape, about without flagella. Their chloroplast contains the green photosynthetic pigments chlorophyll-a and b. Chlorella has different ecology values also as it is used in food culture and in medicines.
Option D is not correct.

Hence, Option C is the correct answer.

Note:
As we discussed about the coenobium as it is a colonial alga, it is a type of Volvox. It can be single cup shaped chloroplasts. Each individual cell is attached to each other with cytoplasmic strands. The cells of Volvox swim in a coordinated fashion, with distinct anterior and posterior poles. The cells have anterior eyespots and also posterior poles. Volvox is also used in ecological cultures.