The correct answer is B. Gracilaria.
Agar is a gelatinous substance extracted from red algae. It is used as a gelling agent in a wide variety of foods, cosmetics, and pharmaceuticals. Gracilaria is a type of red algae that is particularly well-suited for the production of agar. It is a fast-growing algae that can be cultivated in a variety of marine environments. Agar from Gracilaria is a high-quality product that is used in a variety of applications.
Chlorella is a type of green algae that is often used as a food supplement. It is a single-celled organism that is rich in protein, vitamins, and minerals. Chlorella is not a good source of agar.
Parphyra is a type of red algae that is used in some Asian cuisines. It is a flat, sheet-like algae that is often dried and then rehydrated before cooking. Parphyra is not a good source of agar.
Volvox is a type of green algae that forms spherical colonies. It is a common organism in freshwater habitats. Volvox is not a good source of agar.