IMO (D)

This one is tricky .

It wants us to believe that a false comparison is being tested, and we need to correct for it. However, we are not comparing 'apartment buildings' to 'rooms.' Nonetheless, we can eliminate those answer choices that make a false comparison: (A), (C).

(B) is wordy, 'apartments that are in buildings...'
(E) is also is a little word with the 'have...in them' is a little wordy. (D) is more concise.

Unlike smaller apartment buildings, which have fewer than four residential units in them, each room within any residential complex must be equipped with a sprinkler system.

(A) Unlike smaller apartment buildings, which have fewer than four residential units in them
(B) Apart from those apartments that are in buildings that contain fewer than four residential units
(C) In contrast to smaller apartment buildings that contain fewer than four residential units
(D) Unless the apartment building contains fewer than four residential units ANSWER
(E) Excluding those apartment buildings that have fewer than four residential units in them