A. Propanal (CH3CH2CHO) and propanone (CH3COCH3): Same formula C3H6O, different functional groups (aldehyde vs ketone) → Functional isomers. True
B. Ethoxyethane (C2H5OC2H5) and methoxypropane (CH3OC3H7): Same ether group, different alkyl groups → Metamers. True
C. But-2-ene (CH3CH=CHCH3): No chiral carbon, shows geometrical isomerism (cis-trans), not optical. False
D. But-1-ene and but-2-ene: Both are alkenes, differ in position of double bond → Position isomers, not functional isomers. False
E. Pentane and 2,2-dimethylpropane: Both C5H12, different carbon skeleton → Chain isomers. True
Correct: A, B and E only.