Algae with motile colony is

The correct option is Volvox.

Let's understand why this option is correct by breaking down the concepts step-by-step:

1. What is a colony in algae?
A colony is an association of individual cells of the same species that live together in a coordinated manner. In some colonial algae, the colony has a fixed number of cells arranged in a specific pattern, which is known as a coenobium.

2. What makes a colony motile?
A colony is considered motile if its individual cells possess flagella (whip-like structures used for movement) that beat in a coordinated fashion, allowing the entire colony to swim or roll through the water.

3. Analyzing the given options:

• Nostoc: It is a genus of cyanobacteria (blue-green algae) that forms filamentous colonies wrapped in a gelatinous sheath. These colonies are non-motile.
• Chlamydomonas: It is a unicellular, motile green alga. Although it has flagella and is motile, it exists as a single cell, not as a colony.
• Spirogyra: It is a filamentous green alga. Its cells are joined end-to-end to form unbranched filaments, which are non-motile.
• Volvox: It is a multicellular, hollow spherical colony (coenobium) made up of thousands of flagellated cells. The coordinated beating of these flagella enables the entire colony to exhibit rolling motility. Therefore, Volvox is a classic example of a motile colonial alga.