Read the Passage carefully and answer the Questions.

Carbonyl compounds such as aldehydes, ketones, and carboxylic acids play an important role in the biochemical process of plants and animal kingdom. Many aldehydes and ketones find applications in dye, perfumery and other industries. The carbonyl carbon atom is $sp^2$-hybridized forming three sigma and one pi-bond with oxygen. C-O bonds are polar bond due to high electronegativity of oxygen than carbon. Hence the carbonyl carbon behaves as an electrophilic center and oxygen as a nucleophilic centre. Primary and secondary alcohols yield aldehydes and ketones respectively upon oxidation with variety of oxidizing agents. The boiling points of aldehydes and ketones differ from hydrocarbons and ethers. Aldehydes and ketones undergo nucleophilic addition reaction due to presence of electrophilic carbonyl carbon. Nucleophile and a proton add across the carbon oxygen double bond.

Formalin which is used to preserve biological specimen is an aqueous solution of

40% Formaldehyde

The correct answer is Option (1) → 40% Formaldehyde

Formalin, the preservative used for biological specimens, is a commercial aqueous solution containing approximately 37–40% formaldehyde (by weight/volume), often stabilized with 10–15% methanol. This is the standard concentrated form referred to as "formalin" or "100% formalin."

• The diluted version (e.g., 10% formalin) contains about 4% formaldehyde and is commonly used for fixation/preservation in labs.
• Among the options, 40% is the closest and standard description for formalin itself.