Room/day is the cost unit used in . . . . . . . .

The correct answer is: A. hotels

Room/day is a cost unit used in hotels to calculate the cost of a room for one day. It is calculated by multiplying the room rate by the number of days that the room is occupied. For example, if the room rate is $100 per night and the room is occupied for 3 nights, the cost of the room would be $300.

The other options are incorrect because they are not typically used as cost units in hotels. Hospitals typically use cost units such as patient day or case mix index. Schools typically use cost units such as student day or teacher salary.