Which of the following algae contains mannitol as reserve food material ?

The correct answer/option is Ectocarpus.

To understand why, let us analyze the storage food materials in different groups of algae:

1. Brown Algae (Phaeophyceae): Members of this class store reserve food material in the form of complex carbohydrates, specifically laminarin or mannitol (a sugar alcohol). Ectocarpus is a filamentous brown alga, and therefore it contains mannitol as a reserve food material.
2. Red Algae (Rhodophyceae): Members of this class, such as Gracilaria, store reserve food in the form of floridean starch, which is structurally very similar to amylopectin and glycogen.
3. Green Algae (Chlorophyceae): Members of this class, including Volvox and Ulothrix, store reserve food material in the form of starch.

Therefore, among the given options, only Ectocarpus contains mannitol as its reserve food material.