Question

Isomers of propionic acid are:
(A)$\text{HCOO}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\,\,\,\text{and}\,\,\,\text{C}{{\text{H}}_{\text{3}}}\text{COOC}{{\text{H}}_{\text{3}}}$
(B)$\text{H-COO}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\,\,\,\text{and}\,\,\,{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{7}}}\text{COOH}$
(C)$\text{C}{{\text{H}}_{\text{3}}}\text{COOC}{{\text{H}}_{\text{3}}}\,\,\,\,\text{and}\,\,{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{7}}}\text{OH}$
(D)${{\text{C}}_{\text{3}}}{{\text{H}}_{\text{7}}}\text{OH}\,\,\text{and }\,\text{C}{{\text{H}}_{\text{3}}}\text{COC}{{\text{H}}_{\text{3}}}$

Answer 1

Two or more than two organic compounds having the same molecular formula and molecular weight but they differ in physical and chemical properties are called isomers.

<>Complete answer:**
Functional isomers are a kind of structural isomers in which two organic compounds have the same molecular formula but they differ in functional groups. There are several examples of functional isomerism such as Alcohol show functional isomerism with ethers and aldehyde show functional isomerism with ketone. Acid are functional isomers of ester.
Acids shows functional isomerism with esters, where in acids functional group is carboxylic group $\text{(-COOH)}$ in which a carbonyl group $\left( \text{-CO} \right)$ is attached with a hydroxyl group $\text{(-OH)}$. While in ester functional group is $\text{(-COOR)}$ in which a carbonyl group $\left( \text{-CO} \right)$ is attached with the alkoxy group $\text{(-OR)}$.
(A)Propionic acid $\text{(C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}\text{COOH)}$ is a three carbon compound which show functional isomerism with methyl acetate $\text{(C}{{\text{H}}_{\text{3}}}\text{COOC}{{\text{H}}_{\text{3}}})$. Both the compounds have the same molecular formula and molecular weight but differ in functional group.
(B) Propionic acid shows functional isomerism with methyl acetate, but does not show isomerism with butanoic acid because they do not have the same molecular formula and molecular weight.
(C) Propionic acid does not show functional isomerism with ester but not with alcohol.
(D) Propionic acid does not show functional isomerism with alcohols and ether.

Hence the option (A) is the correct answer.

Note: Alcohols show functional isomerism with ether only. In the same way alcoholic and phenolic groups are functional isomers.
If the functional group is different in two compounds they will never show other structural isomerism.