As a result of the Black Death, the plague that afflicted Europe in 1348-49, as many as half the population died and less than half the work force remained in certain cities.
as much as half the population died and less than half the work force remained
First of all, population is not a quantity than can be counted so as many as needs to be changed to as much as. Eliminate (A).
Option (C) survives this particular comparison because it changes the subject from population to the countable people and therefore uses as many as correctly. As for the second quantity in the underlined portion, work force is, like population, not countable and therefore less than rather than fewer than is correct. Eliminate (B). (C) is wrong because less than that remained distorts the meaning of the sentence and isunclear in its own right. What exactly is that? Half of those who died? Who survived? Thus, option (D) is correct.