C4H6O structural isomers with groups for 1:1 HCN nucleophilic addition (carbonyls, epoxide) Steps:

• Compute unsaturation: (2×4 + 2 - 6)/2 = 2 degrees, consistent with one C=O or epoxide plus one double bond/ring.
• Identify reactive groups: aldehydes/ketones add HCN to C=O forming cyanohydrin; epoxides undergo ring-opening addition.
• Enumerate aldehydes (4): but-3-enal (CH₂=CHCH₂CHO), but-2-enal (CH₃CH=CHCHO), 2-methylprop-2-enal (CH₂=C(CH₃)CHO), cyclopropanecarboxaldehyde.
• Enumerate ketones (2) and epoxide (1): but-3-en-2-one (CH₃COCH=CH₂), cyclobutanone, 2-vinyloxirane (epoxybutene); total 7 isomers. Why C is correct:
• Exactly 7 structural isomers possess C=O or epoxide for 1:1 nucleophilic addition per the reaction specification. Why the others are wrong:
• A. 5: Excludes epoxide and one unsaturated aldehyde.
• B. 6: Counts only carbonyl isomers, omitting epoxide.
• D. 8: Includes stereoisomers (e.g., E/Z-but-2-enal), violating structural-only constraint.

Final answer: C