Which of the following pairs is of unicellular algae ?

Chlorella and Spirulina are unicellular. Chlorella is unicellular green algae and Spirulina is unicellular blue green algae (cyanobacteria).

Chlorella a unicellular alga rich in proteins is used as food supplement even by space travelers. Spirulina can serve as food rich in protein, minerals, fats, carbohydrate and vitamins and can be used as SCP.

Gelidium and Gracilaria are red algae and they are multicellular.  Laminaria and Sargassum are multicellular brown algae. Anabaena is filamentous multicellular cyanobacteria and Volvox is a colonial green algae.