Choice A is correct. A broad signal near 2850 cm"1 indicates the hydroxyl group of a carboxylic acid (O-H with hydrogen-bonding). A hydroxyl group exhibits hydrogen-bonding, so the signal is broad. Because the O—H bond of a carboxylic acid is weak, its absorbance is lower than that of standard hydroxyl groups. Of the answer choices, only choice A has a carboxylic acid functionality, let alone a hydroxyl group. Pick A for happiness.
I see the answer, and I suppose it makes sense, but I was wondering how I was supposed to conclude that. The only information from passage about signals below 3000 (except the info about 1715) is the following:
If a carbon has sp3-hybridization, the bonds it forms to hydrogen are found just below 3000 cm'1.
I thought that phrase was talking about a CH3.