Algae

The correct answer is option (A): Volvox

The algae that forms a motile colony is Volvox. Volvox is a type of green algae that consists of individual cells working together in a spherical colony. These cells are interconnected by thin strands of cytoplasm and can move as a coordinated unit through the use of flagella. This motility allows Volvox colonies to move in a coordinated manner in water bodies. The other options listed, Nostoc, Spirogyra, and Chlamydomonas, do not form motile colonies like Volvox does.

Agar-agar is a jelly-like substance that is a non-nitrogenous carbohydrate. It consists of two polysaccharides - agarose and agaropectin.

• Agar agar is obtained from red algae like Gracillaria, Gefidiurn, and Gigartinia.
• It was discovered by Lady Hesse and used by Robert Koch to solidify culture medium.