Ectocarpus is the correct answer as it is a type of algae that contains mannitol as a reserve food material. Mannitol is a type of sugar alcohol that serves as a carbohydrate storage molecule in certain algae.

Explanation:

Algae:
Algae are a diverse group of photosynthetic organisms that can be found in various aquatic environments including marine, freshwater, and even damp soil. They are classified as a part of the Kingdom Protista and are typically simple, plant-like organisms that lack roots, stems, and leaves.

Ectocarpus:
Ectocarpus is a genus of brown algae, also known as Phaeophyta. It is commonly found in marine environments, particularly along rocky shores. Ectocarpus is filamentous in structure, with branching filaments that can form tufts or mats. It has a complex life cycle that alternates between a haploid gametophyte stage and a diploid sporophyte stage.

Mannitol:
Mannitol is a type of sugar alcohol that is commonly found in algae. It serves as a reserve food material, providing energy for the organism when needed. Mannitol is a non-reducing sugar and is often used as a storage form of carbohydrates in algae.

Other options:
- Gracilaria: Gracilaria is a genus of red algae, also known as Rhodophyta. It is commonly found in marine environments and is often used for agar production.
- Volvox: Volvox is a genus of green algae, also known as Chlorophyta. It is characterized by its spherical colony structure and is often found in freshwater environments.
- Ulothrix: Ulothrix is a genus of green algae, also known as Chlorophyta. It is filamentous in structure and is commonly found in freshwater environments.

Conclusion:
Among the given options, Ectocarpus is the algae that contains mannitol as a reserve food material. Mannitol serves as a carbohydrate storage molecule in algae, providing energy when needed.