The general formula ${{C}_{n}}{{H}_{2n}}{{O}_{2}}$ could be for open chain
A. Diketones
B. Carboxylic acid
C. Diols
D. Dialdehydes

Hint: A general formula is a type of empirical formula that represents the composition of any member of an entire class of compounds. In chemistry, a chemical compound's empirical formula represents the simplest positive integer ratio of atoms present in a compound.

Complete answer:
A general formula is a type of empirical formula that represents the composition of any member of an entire class of compounds.
A. Diketone: A diketone or dione is a molecule containing two ketone groups. The simplest diketone is diacetyl, also known as 2,3-butanedione ${{(C{{H}_{3}}CO)}_{2}}$. Diacetyl, acetylacetone, and hexane-2,5-dione are examples of 1,2-, 1,3-, 1,4-diketones, respectively. Hence, the general formula of diketones is ${{C}_{n}}{{H}_{2n-2}}{{O}_{2}}$.
B. Carboxylic acid: A carboxylic acid is an organic compound that contains a carboxyl group (-COOH) attached to an R- group. The general formula of carboxylic acid is R-COOH, with R referring to the alkyl group. Important examples include the amino acids and fatty acids. Deprotonation of carboxylic acid gives a carboxylate anion. Examples of carboxylic acid are Methanoic acid (HCOOH), Ethanoic acid $C{{H}_{3}}COOH$ , Propanoic acid $C{{H}_{3}}C{{H}_{2}}COOH$ etc. Hence, the general formula of carboxylic acid is ${{C}_{n}}{{H}_{2n}}{{O}_{2}}$.
C. Diols: A diol is a chemical compound containing two hydroxyl groups (-OH groups). An aliphatic diol is also called a glycol. The most common industrial diol is ethylene glycol. Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediol $OH-{{\left( C{{H}_{2}} \right)}_{4}}-OH$ and propylene-1,3-diol, or beta propylene glycol, $OH-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{2}}-OH$. Hence, the general formula of diols is ${{C}_{n}}{{H}_{3n}}{{O}_{2}}$ .
D. Dialdehydes: A dialdehyde is an organic chemical compound with two aldehyde groups. The nomenclature of aldehydes have the ending -dial or sometimes- dialdehyde. Examples of aldehydes are butanedial, which is also called succinaldehyde ${{(C{{H}_{2}}CHO)}_{2}}$, malondialdehyde $\left( C{{H}_{2}}{{\left( CHO \right)}_{2}} \right)$ etc. Hence, the general formula of aldehydes is ${{C}_{n}}{{H}_{n}}{{O}_{2}}$.
Hence, the general formula ${{C}_{n}}{{H}_{2n}}{{O}_{2}}$ could be for open chain of carboxylic acid.

Hence, option B is correct.

Note:
Always remember, the general formula of carboxylic acid is ${{C}_{n}}{{H}_{2n}}{{O}_{2}}$ for open chain.
Always remember, the general formula of diols is ${{C}_{n}}{{H}_{3n}}{{O}_{2}}$ for open chain.
Always remember, the general formula of diketones is ${{C}_{n}}{{H}_{2n-2}}{{O}_{2}}$ for open chain.