I can give you four answers to this question. The short answer, the long answer, the historical answer, the actual answer, and the mythbusters recreate-the-myth answer.
probably C:, if not D:, but could be anything up to Z:
It depends entirely on which version of DOS this machine uses.
Originally DOS assigned all floppy drives first, then hard drives, then network drives. So a system with three floppy drives and one hard drive would get floppy drives A:, B:, C:, hard drive D:
DOS 5.0 (from 1991) changed it, and it now does the first two floppy drives, then the primary partition of hard drives, then all the extra partitions of hard drives, THEN floppy drives from 3 and up.
So a single hard drive machine (assuming it's not partitioned into multiple drives) with three floppy drives, will have floppy drives A:, B:, and D:, with hard drive C:.
It originally worked this way because DOS copied how CP/M did it, and CP/M just assigned the letters A-Z to all drives, without treating hard drives specially (especially because you probably didn't have a hard drive back then)
The floppy drive is A:. This machine only has one floppy drive.
This case has blanking plates for the 3.5" floppy drive bays that look like fake 3.5" floppy drives. I don't know why it does this, it's very weird and confusing, and in this picture they've installed neither of the possible 3.5" floppy drives, so both of those are fake, and only the 5.25" drive is real.
mythbusters recreate-the-myth answer:
I went into 86box and set up an IBM XT with three floppy drives, and installed DOS 3.3 and then DOS 5.0 on it:
Here in DOS 3.3, you can see that our floppy drives got numbered A:, B:, and C:, and the hard drive is D:
But when that same hardware is upgraded to DOS 5.00, the hard drive is C:, and the floppies are A:, B:, and D:
turnnoffyourmind
dededos
kianamaiart
tangentwithtanner
