Actually, "amounting" is not a tensed verb at all. It's a present participle -- a grammatical object formed from a verb but that can take on a number of grammatical functions. Here, it's being used to start a modifying phrase. The essential question: what is it modifying?
If the retailers do not pass on the discount to their customers, then there is no reason for the manufacturers to reduce profit margins and offer the retailers discount. They are better off making higher profits by not offering discounts.
C, increase had been slight (for males).......great (for females) - for 引导修饰slight/great的状语， 应该 increase (in females / males)修饰 increase.
Had been 时态错误
E: if you say "the last day that they are scheduled to collect", this means that "day" must be a direct object.
this makes no sense, unless the employees are collecting days.
"the time that we were scheduled to meet" is incorrect. the literal reading of this (which is the only reading you care about, by the way) is that you were scheduled to meet the time itself.
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E uses the relative pronoun "which." As written, its antecedent would be "years" and that makes no sense in the sentence.
C and D fix the problem by introducing "an increase." Now, "amounting to..." is a noun modifier that modifies "an increase." Eliminate A, B, and E.
Between C and D, eliminate C because it introduces an ambiguous pronoun "it." I'd go for D.