Which of the following compounds are isomers of propionic acid?

Which among the given molecules can exhibit tautomerism?

Which of the following organic structures is $NOT$ an isomer of the compound $CH_3COCH_2CH_2CH_2CH_3$?

Assertion $A$: Enol form of acetone $[CH_{3}COCH_{3}]$ exists in $< 0.1 \%$ quantity. However,the enol form of acetylacetone $[CH_{3}COCH_{2}COCH_{3}]$ exists in approximately $15 \%$ quantity.
Reason $R$: Enol form of acetylacetone is stabilized by intramolecular hydrogen bonding,which is not possible in the enol form of acetone. Choose the correct statement:

The number of structural isomers for the molecular formula $C_{4}H_{10}O$ is:

The number of structural isomers for $C_6H_{14}$ is