Explanation

In this sentence, Mr. Vincente did something in a way more harsh than his predecessors. The key word here is smoother. We need two words that go in the opposite direction.

B and D are the best and similar in their meaning we need for the purpose.
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Since becoming commissioner, Mr. Vincente has snapped at the heels of the dominant firms in European industry more __________ than his smoother predecessors and has consequently acquired many more enemies.

He has snapped at the heels of the dominant firms to a greater extent than his smoother predecessors. We know this because he acquired many more enemies. But we need more precise words, "greater extent" is too general and gives us only the magnitude, which we know was greater.

The phrase "smoother predecessors" implies he was kind of rough.

So irascibly and pugnaciously are the right choices. They are not exactly synonyms, but they are the only ones which convey a greater aggression implied by the sentence, especially by contrast, through the phrase "smoother predecessors".
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The blank needs to be in contrast with the smoothness of his predecessor, as he also made more enemies implies that he was harsh and acerbic. Hence, B and D

I don't see how BD are synonyms. Irascibly means easily angered while pugnaciously means quarrelsome (not the same connotation).

I assumed it would be AE, sporadically and fitfully, because they are perfect synonyms and because of the contrast from the phrase "smoother predecessors". The opposite of smooth is rough but it can also be irregular which is what AE mean.

Someone, please explain the fallacy in my reasoning.

Ok let me clear so you understand perfectly how SE works

Basically ETS creates the answer choices in the SE questions in pair

2 pairs of synonyms and one spurious words: 2+2+1-1. rarely 2+2+2

In this case we do have A and E, B and D, and C and F are not matched

between the pairs A and E / B and D the former are out, clearly

B and D mean aggressive which is the meaning of the blank

B and D are the answers

See also my reply to you here yesterday https://greprepclub.com/forum/internati ... tml#p63979
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