First, eliminate (A) and (B) because of vague pronoun use. In those choices, "it" could refer to either the decrease or "retail inventory." (C) is incorrect, as "what would be" refers to something hypothetical, while the rest of the sentence indicates that the decrease has actually happened.

Choice (D) is incorrect as well. A "six-year low level" is vague; it could refer to a level that will be low for six years as well as something more in line with the intended meaning of the sentence. That leaves choice (E), which is correct. The meaning is clear, and it avoids the pronoun problems of the first two choices.