[cockleivory]

I can't figure out if the answer is (a) or (b). As I understand, the frequency of sp3C-H bond is around 2850-2950; that of sp2C-H is around 3000-3100. I know the answer is not (c) due to the 1700 peak that stands for the carbonyl group. But I do not know how to interpret the peaks at around 3000.

[pgk]

Such an IR spectrum around 3000cm-1, indicates strong intramolecular hydrogen bonds. According your opinion, which one creates the strongest intramolecular hydrogen bonds (a), (b) or (c) and why?

[cockleivory]

(b), then. Thank you for the extra information about the intramolecular HB. It's very useful to know.

[pgk]

This is a typical IR spectrum of a carboxylic acid. By dilution of concentration, the band around 3000 cm-1 changes its appearance. Contrary, IR appearance does not change by concentration or dilution, when intermolecular hydrogen bonds exist.
For the same reason, an IR spectrum of (a) that contains humidity would have not the same but a similar appearance with that of (b).