Which room in the Baths of Diocletian is designated for dressing?

The Apodyterium is the room designated for dressing in the Baths of Diocletian, as well as in other Roman bath complexes. This space serves as the entrance and provides a space for bathers to undress and store their clothes in niches or cupboards. The design of the Apodyterium is functional, accommodating the social customs of Roman bathing which emphasized cleanliness and relaxation.

In comparison, the Caldarium is the hot bath area, characterized by its hot water and steam, while the Frigidarium is the cold bath. The Tepidarium serves as a warm room that acts as a transition between the hot and cold baths. Each of these spaces plays a specific role in the bathing process, but the Apodyterium is distinctive for its function associated with dressing and undressing, thus making it the correct answer.