Which of the following compounds would show only one triplet in its off resonance decoupled spectrum?

A) acetone
B) butanal
C) pentanal
D) 2-pentanone
E) 3-pentanone

In off-resonance decoupling, all proton-proton scalar couplings are removed. As a result, each set of chemically equivalent protons will appear as a single peak in the spectrum. Since the question asks for a compound that shows only one triplet, we need to look for a compound that has only one set of chemically equivalent protons that are coupled to each other with a scalar coupling constant of J.

Acetone has three sets of chemically equivalent protons: the methyl group and the two carbonyl groups. Each set would give a triplet and therefore we would see three triplets in the spectrum.

Similarly, butanal has two sets of chemically equivalent protons: the methyl group and the methylene group. Each set would give a triplet and therefore we would see two triplets in the spectrum.

Pentanal has three sets of chemically equivalent protons: the two methyl groups and the methylene group next to the carbonyl group. Each set would give a triplet and therefore we would see three triplets in the spectrum.

2-pentanone has two sets of chemically equivalent protons: the methyl group and the methylene group next to the carbonyl group. Each set would give a triplet and therefore we would see two triplets in the spectrum.

3-pentanone has only one set of chemically equivalent protons that are coupled to each other with a scalar coupling constant of J. Therefore, we would see only one triplet in the spectrum.