In Dinoflagellates, the reserve food is

The correct option is Starch.

Step-by-step Explanation:

1. Understanding Dinoflagellates:
Dinoflagellates are a large group of single-celled eukaryotic organisms belonging to the phylum Dinoflagellata. They are mostly marine plankton, but they can also be found in freshwater habitats. A significant portion of dinoflagellates are photosynthetic autotrophs, meaning they synthesize their own food using light energy.

2. Photosynthetic Pigments and Food Production:
Photosynthetic dinoflagellates possess chloroplasts containing pigments like chlorophyll a, chlorophyll c, and various carotenoids (such as peridinin). Through the process of photosynthesis, they capture light energy and convert inorganic carbon dioxide and water into organic energy-rich molecules (carbohydrates).

3. Reserve Food Material:
The primary carbohydrate synthesized and stored as reserve food in dinoflagellates is starch, which is stored outside the chloroplasts (in the cytoplasm), often surrounding the pyrenoid if present. Additionally, they may store reserve energy in the form of oil droplets or lipids.

4. Analyzing Other Options:
- Fucoxanthin: This is a carotenoid pigment responsible for the brown color in brown algae (Phaeophyceae) and diatoms, not a storage food.
- Mannitol: This is a sugar alcohol that serves as a primary reserve food in brown algae (Phaeophyceae), along with laminarin.
- Alginic acid: This is a structural polysaccharide found in the cell walls of brown algae, not a storage food.

Therefore, starch is the characteristic reserve food material stored by Dinoflagellates.