Basement 2, B2, second floor underground

A: Which floor is your classroom?
B: It’s on the 1) basement two 2) B2 3) second floor underground.

May I know which one do you usually use?

Thank you!!

“It’s in the basement, B2.” Would probably be my response if I was standing near the elevator.

The question should be posed, “on which floor is your classroom” or (more informally), “which floor is your classroom on”? Grammatically, one should not end a sentence with a preposition like “on, to, of” but in informal speech it is common to do so. Whichever version of the question you use, the question still requires “on.” Without it, the question means that the entire floor comprises the classroom which is generally not the case. Rather, you want to know ON which floor the classroom is located.

Regarding how floors are identified, there is a lot of variation. Any underground floor in a building would be “in the basement” but such floors can be identified in various ways. Just yesterday I went to a commercial building with two underground levels: LL1 and LL2. (LL=lower level). I have also seen “C” used to designate an underground space in a commercial complex which means “common area.” “B” (for basement) is only one of several ways to identify “below grade” space. I would not expect to see “B2” in reference to a basement space unless there were at least two underground levels (B1 and B2).

By the way, underground spaces in parking lots, train stations and some commercial establishments are more frequently referred to as “levels,” not “floors.” A question regarding the location of something in such cases still requires the word, “on” – i.e., “on which level is your car parked?” “It is parked on LL2.” “On which level is the D train?” “It is located on LL1.” “Where is the food court?” “It is on level C.”

If a building only has one underground space, then one could say, “the classroom is in the basement.” However, if the building has more than one underground level, then it is necessary to ask, “on which floor in the basement is the class located?” It is located on ___ [floor or level]…"