To solve this problem, we need to identify which alga can be used as food for human beings.$\newline$Let's analyze each option:$\newline$• Option 1: Ulothrix - This is a filamentous green alga, not commonly used as food for humans.$\newline$• Option 2: Chlorella - This is a single-celled green alga known for its high protein content and is used as a dietary supplement.$\newline$• Option 3: Spirogyra - This is a filamentous green alga, not typically consumed by humans.$\newline$• Option 4: Polysiphonia - This is a red alga, not commonly used as food for humans.$\newline$Chlorella is rich in proteins, vitamins, and minerals, making it suitable for human consumption.$\newline$Therefore, the correct option is Option 2: Chlorella.