Coenobium refers to

The correct option is A hollow spherical colony.

To understand why this is the correct answer, let us break down what a coenobium is:
A coenobium (plural: coenobia) is a specialized type of colony found in certain algae (such as Volvox or Scenedesmus). It is characterized by having a fixed number of cells that are arranged in a specific, organized pattern. Once the coenobium is formed, no new cells are added, and the cells grow only in size rather than number during its life cycle.
In many classic examples, such as the green alga Volvox, the coenobium is structured as a hollow sphere (or ellipsoid) where individual flagellated cells are arranged in a single outer layer, connected to each other by cytoplasmic strands. Therefore, the term "coenobium" specifically refers to this hollow spherical colony structure.