To solve this problem, we need to identify which pair consists of unicellular algae. Let's analyze each option:$\newline \bullet$Option 1: Gelidium and Gracilaria$\newline$ - Gelidium and Gracilaria are both genera of red algae, which are typically multicellular.$\newline$ - Therefore, this option does not represent unicellular algae.$\newline \bullet$Option 2: Anabaena and Volvox$\newline$ - Anabaena is a genus of cyanobacteria, which are prokaryotic and not considered true algae.$\newline$ - Volvox is a genus of green algae, but it is colonial, not unicellular.$\newline$ - Therefore, this option does not represent a pair of unicellular algae.$\newline \bullet$Option 3: Chlorella and Spirulina$\newline$ - Chlorella is a genus of green algae that is unicellular.$\newline$ - Spirulina is a genus of cyanobacteria, often referred to as blue-green algae, and is also unicellular.$\newline$ - This option correctly represents a pair of unicellular algae.$\newline \bullet$Option 4: Laminaria and Sargassum$\newline$ - Laminaria and Sargassum are both genera of brown algae, which are multicellular.$\newline$ - Therefore, this option does not represent unicellular algae.$\newline$Based on the analysis, the correct answer is Option 3: Chlorella and Spirulina, as they are both unicellular algae.