Hi!
Scanning through the choices, we can see that there are two big issues in this question:
1) the comparison at the beginning; and
2) the verb tense at the end.
Regarding the comparison, all comparisons must be both logically and stylistically parallel. In the original sentence, we compare "recent amendments to the rules governing baseball" to "those governing professional gymnastics"; in other words, we're comparing amendments to rules, which is illogical. Eliminate (A).
Scanning the choices:
B) compares amendments to "those to the rules", i.e. other amendments - all good!
C) compares amendments to "those to the rules", i.e. other amendments - all good! However, (C) changes "governing" to "which govern", disrupting the parallelism of the sentence (the first part of the comparison has "governing") - eliminate (C).
D) compares "the amendments" to "the amendments" - all good!
E) compares "amendments to the rules" to "amendments governing professional gymnastics" - the amendments don't govern, the rules do - eliminate (E).
Now let's scan (B) and (D) for another difference. (B) ends in "were motivated" and (D) ends in "have been motivated". Are there any clues in the original to make us think this is an ongoing action? No, in fact we have two reasons to believe that the action is discrete:
1) "the recent amendments" is all in the past; and
2) the original sentence uses "were motivated".
Changing the verb to "have been motivated" changes the meaning of the sentence, a big no-no in SC. Accordingly, eliminate (D) and choose (B)!