Ulothrix is a green alga (Chlorophyceae) and stores its reserve food as starch, not mannitol. Mannitol is the storage product of brown algae (Phaeophyceae).

The other pairings are correct: Ectocarpus (Phaeophyceae) contains fucoxanthin, Porphyra (Rhodophyceae) stores floridean starch, and Volvox (Chlorophyceae) stores starch.