Which sugar has a hydroxyl group attached to the 2' carbon?

Ribose is the correct answer because it is a five-carbon sugar (pentose) that contains a hydroxyl group (-OH) attached to its 2' carbon atom. This characteristic is significant because it differentiates ribose from deoxyribose, which lacks the hydroxyl group at the 2' position and instead has a hydrogen atom. This absence of the hydroxyl group is what gives deoxyribose its name, indicating that it is a "deoxygenated" form of ribose.

In contrast, galactose and glucose are both six-carbon sugars (hexoses) and do not have the same configuration around the 2' carbon as ribose does. Their structural characteristics do not include the defining feature of ribose’s hydroxyl attachment at the 2' position. Thus, when considering sugars with a hydroxyl group on the 2' carbon, ribose stands out as the only correct option.