In the United States, less than half as many multifamily housing units were produced in the 1990s than in each of the previous two decades.

Here is my take. We are not saying housing units in 1990 is more or less than that of the last two decades. We are saying housing was equal. hence use of "AS" is appropriate

Also, fewer than is correct since housing units is countable


A. less than half as many multifamily housing units were produced in the 1990s than
Than is incorrect

B. less than half as many multifamily housing units had been produced in the 1990s as
less than is Incorrect
Than is incorrect

C. there were less than half as many multifamily housing units produced in the 1990s than
Than is incorrect
than is Incorrect

D. fewer than half as many multifamily housing units were produced in the 1990s as
Correct

E. fewer than half as many multifamily housing units had been produced in the 1990s than
than is Incorrect