My answer is
(D).
In reading the sentence (i.e. option A), I found a fatal error: "it“ cannot logically be the agent who ”calculates the tax". Armed with such revelation, I can immediately eliminate all but (C) (and already started to wonder why the split is so extreme.) Upon careful reading of (C), I realize that it also use "when calculating the tax" with a logical subject "it".
So, this question writer totally disregards the compatibility between sentence subject and participles used as modifier. I have to ignore this issue to re-examine all options, after such a thrill ride.
(A) "annual" and "at the end of the year" is redundant. On the other hand, should the annual tax is due in the middle of the year, we can argue that "annual" and "in the middle of the year" may not be redundant.
(B) "if" can only be used to introduce a conditional subordinate clause in GMAT. Must use "whether" here to introduce a noun clause.
(C) "whether or not" is redundant. "whether" suffices.
(E) "whether ... or not" is redundant. "whether" suffices.