Lets begin by looking at the overall structure of the sentence:
Merchandisers reported mild profits in their December business, as much because of their top line revenue of a year earlier being so weak as that
customers were gaining a new beginning on purchasing their Christmas present."as much"
is an unfinished idiom so we must complete it keeping the parallel strcuture: as much because (...) as because. Only B C D have "because" in the final part, so lets keep them.
(B) of their top line revenue of a year earlier having been as weak as because
(C) of their a top line revenue of the year before being as weak as because
(D) their top line revenue a year before had been so weak as because
However the parallelism in B and C is flawed at the beginning: as much because of
(...) as because. Only D has the perfect parallelism hence is the correct answer.
Hope it's clear
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