Basics of Organic Chemistry — JEE Main practice

1. Check statement A. Since propanal (aldehyde) and propanone (ketone) share the molecular formula $\mathrm{C}_3\mathrm{H}_6\mathrm{O}$ but have different functional groups, they are indeed functional isomers. Statement A is true.

2. Check statement B. Given ethoxyethane (diethyl ether) and methoxypropane both have the ether functional group with the same molecular formula but differ in how the alkyl groups are distributed around the oxygen, they are metamers. Statement B is true.

3. Check statement C. Since but-2-ene is a planar alkene with a plane of symmetry and no chiral center, it does not show optical isomerism (it shows only cis-trans geometric isomerism). Statement C is false.

4. Check statement D. Given but-1-ene and but-2-ene both have the same functional group (alkene) differing only in double bond position, they are position isomers, not functional isomers. Statement D is false.

5. Check statement E. Since pentane (straight chain) and 2,2-dimethylpropane (branched, same molecular formula $\mathrm{C}_5\mathrm{H}_{12}$) differ in their carbon skeleton, they are chain isomers. Statement E is true.

Hence, the correct statements are A, B, and E only — the answer is option A.

1. Identify which compound shows optical isomerism. Since optical isomerism requires a carbon attached to four different groups, checking each candidate: n-propyl chloride, iso-propyl chloride, and neo-pentyl chloride all lack such a carbon, but sec-butyl chloride ($\mathrm{CH}_3-\mathrm{CHCl}-\mathrm{CH}_2-\mathrm{CH}_3$) has its C2 attached to $\mathrm{H}$, $\mathrm{Cl}$, $\mathrm{CH}_3$, and $\mathrm{CH}_2\mathrm{CH}_3$ — four different groups, making it chiral.

2. Determine the molecular formula of sec-butyl chloride. Since it's $\mathrm{C}_4\mathrm{H}_9\mathrm{Cl}$,

3. Calculate the percentage of carbon. Given

Hence, rounding gives approximately 52% — the answer is option C.