NEET (UG) 2025 — Question 79

For $\ce{C4H8O}$, degree of unsaturation is $(2\times4 + 2 - 8)/2 = 1$, so every isomer contains one ring. Possible ring sizes: oxirane (3-membered, $\ce{C2H4O}$) plus two extra carbons; oxetane (4-membered, $\ce{C3H6O}$) plus one extra carbon; and tetrahydrofuran (5-membered, $\ce{C4H8O}$). Constitutional isomers from oxirane: $\ce{2-ethyloxirane}$, $\ce{2,2-dimethyloxirane}$, and $\ce{2,3-dimethyloxirane}$. From oxetane: $\ce{2-methyloxetane}$ and $\ce{3-methyloxetane}$; from the 5-membered ring: $\ce{tetrahydrofuran}$. Stereoisomer counts: 2-ethyloxirane = 2 enantiomers; 2,2-dimethyloxirane = 1; 2,3-dimethyloxirane = 3 (one cis/meso form and one trans-enantiomeric pair); 2-methyloxetane = 2 enantiomers; 3-methyloxetane = 1; tetrahydrofuran = 1. Total = $2+1+3+2+1+1 = 10$. Hence the correct option is C (10).