Mannitol is the stored food in 

Mannitol is the stored food in brown algae like Fucus.

Mannitol is a type of sugar alcohol that serves as the primary stored food in Fucus, a genus of brown algae. It acts as a reserve carbohydrate, providing energy and structural support to the algae. Fucus belongs to the group Phaeophyceae (brown algae), which commonly store their food in the form of complex carbohydrates like mannitol and laminarin. These compounds are well-suited for the marine environment in which Fucus thrives, as they help in osmoregulation and energy storage.

The other options, such as Porphyra (red algae), Gracilaria (red algae), and Chara (green algae), do not store mannitol as their reserve food. For example:

• Porphyra and Gracilaria primarily store food as floridean starch.
• Chara stores food in the form of starch.

This distinction highlights the unique metabolic adaptations of Fucus and its reliance on mannitol for survival and growth in its specific ecological niche.