Monocarboxylic acids are functional isomers of:

Functional isomers are compounds that have the same molecular formula but different functional groups. To determine what monocarboxylic acids are functional isomers of, we must compare their molecular formulas with those of the given options.

A monocarboxylic acid has the general formula R-COOH, where R is an alkyl group. For example, acetic acid (CH₃COOH) has the molecular formula C₂H₄O₂. In general, a monocarboxylic acid with n carbon atoms has the molecular formula CₙH₂ₙO₂.

Now, let's examine each option:

Option A: Ethers
Ethers have the general formula R-O-R'. For example, dimethyl ether (CH₃OCH₃) has the molecular formula C₂H₆O. This differs from acetic acid's formula C₂H₄O₂. Therefore, ethers cannot be functional isomers of monocarboxylic acids.

Option B: Amines
Amines have the general formula R-NH₂ for primary amines. For example, ethylamine (CH₃CH₂NH₂) has the molecular formula C₂H₇N. This differs from C₂H₄O₂. Hence, amines cannot be functional isomers.

Option C: Esters
Esters have the general formula R-COO-R'. For example, methyl formate (HCOOCH₃) has the molecular formula C₂H₄O₂, which matches acetic acid's formula. Both have the same atoms but different functional groups: carboxylic acid (-COOH) versus ester (-COO-). Another example: propanoic acid (C₃H₆O₂) and methyl acetate (CH₃COOCH₃, also C₃H₆O₂). Thus, esters are functional isomers of monocarboxylic acids.

Option D: Alcohols
Alcohols have the general formula R-OH. For example, ethanol (CH₃CH₂OH) has the molecular formula C₂H₆O, which differs from C₂H₄O₂. Therefore, alcohols cannot be functional isomers.

Hence, monocarboxylic acids are functional isomers of esters.

So, the correct answer is Option C.