An individual unit in a multi-story rental apartment complex that occupies only one floor is known as a(n):

A studio is a single open-room apartment that combines sleeping, living, and kitchen areas into one space, which describes the internal layout rather than the floor configuration of the unit.

A flat is actually the correct answer; this option was incorrectly labeled as wrong in the question stem's framing, but the correct answer letter per the question is A — however, based on the explanation provided, 'flat' (C) is the intended correct concept, and the answer key letter 'a' appears to be a data discrepancy. Regardless, a penthouse refers to a luxury unit on the top floor of a building and does not define a single-floor unit generically.

A duplex refers to a building or unit that spans two floors connected by an internal staircase, or alternatively a building containing two separate side-by-side units, neither of which matches the single-floor definition.