To solve this problem, we need to identify which algae contains mannitol as a reserve food material.$\newline$Mannitol is a type of sugar alcohol that is used by some algae as a storage carbohydrate.$\newline$Let's analyze each option:$\newline$• Option 1: $$Ectocarpus$\newline$Ectocarpus is a brown alga. Brown algae are known to store carbohydrates in the form of laminarin and mannitol.$\newline$• Option 2: $$Gracilaria$\newline$Gracilaria is a red alga. Red algae typically store carbohydrates as floridean starch, not mannitol.$\newline$• Option 3: $$Volvox$\newline$Volvox is a green alga. Green algae usually store carbohydrates as starch.$\newline$• Option 4: $$Ulothrix$\newline$Ulothrix is also a green alga, which stores carbohydrates as starch.$\newline$Based on this analysis, the algae that contains mannitol as a reserve food material is $$Ectocarpus$.\newline$Therefore, the correct option is Option 1.