The functional group present in an organic acid is:

The definition of an acid is that it consists of molecules that readily dissociate in water and produce H⁺. Consequently, the functional group found in the organic acid will have the same characteristic.

Organic acids contain the functional group -COOH (carboxyl group), which is made up of a carbonyl group (C=O) and a hydroxyl group (-OH). In water, it separates into COO- and H⁺. the following is the response:

$\Rightarrow \mathrm{RCOOH}\rightleftharpoons {\mathrm{RCOO}}^{-}+{\mathrm{H}}^{+}$

The characteristics of the organic acids are a result of this functional group.
The boiling point of these acids is greater than that of alcohols, ketones, or aldehydes with comparable molecular weights, among other physical characteristics of carboxylic acids.
Four-carbon carboxylic acids with a lower molecular weight are miscible in water because they establish hydrogen bonds there.
Thus, "C" is the appropriate response.