Enumerating unbranched ester isomers for C4H8O2

Steps:

• Esters follow RCOOR' where R and R' are unbranched alkyl groups; total carbons = carbons in R + R' + 1 (carbonyl) = 4, so R + R' total 3 carbons.
• Consider R=H (formate ester) with R'=n-propyl (C3H7): HCOOCH2CH2CH3 (propyl formate).
• Consider R=methyl (CH3) with R'=ethyl (C2H5): CH3COOCH2CH3 (ethyl acetate).
• Consider R=ethyl (C2H5) with R'=methyl (CH3): CH3CH2COOCH3 (methyl propanoate).

Why C is correct:

• The molecular formula C4H8O2 matches three distinct unbranched esters from partitioning the alkyl carbons as above.

Why the others are wrong:

• A: Underestimates by ignoring two valid isomers (ethyl acetate and methyl propanoate).
• B: Misses the formate ester (propyl formate) as a valid unbranched option.
• D: Overcounts by including branched structures like isopropyl formate, excluded by "unbranched."

Final answer: C