Formalin is an aqueous solution of :

The correct answer is (c). Formalin is an aqueous solution of formaldehyde, a colorless, flammable, and toxic gas with a strong, pungent odor. It is used as a disinfectant, preservative, and embalming fluid. It is also used in the production of plastics, resins, and other chemicals.

Formic acid is a weak acid that is found in ants and other insects. It is used as a preservative and in the production of other chemicals.

Furfural is a colorless, flammable liquid that is used in the production of resins, solvents, and other chemicals.

Fluorescein is a yellow-green dye that is used in biological research and in the production of fluorescent materials.