The sentence talks about the effects the polar reviews and modest sales had on the author's belief in her own novel.It mentions how she asserted that those without a positive reaction were unable to grasp the nuances of her work. In other words, the polar reviews and modest sales did not shake her belief in her own novel. It did not decrease the belief in her own novel. Therefore, the correct choice is diminish.

why the answer is not belie..?

the meaning of the sentence is that the author, even though the review, did not have to diminish his confidence in his work

Belie means deny. As such, is completely out of scope

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Regards

The polar reviews and modest sales did not.
Belie means to contradict or fail to justify, didn't give the true impression of
The polar reviews and the modest sales did indeed contradict the author's belief that her novel should receive positive reaction.
So belie is not a fit here.

The medium tag is set accordingly with the book.

However, this question is on the easy side.

I do not think who aims to the top score even think about clues, twists, and stuff like that.

Straight be.

For those of who have doubts, I am sorry but they have a long way to go.

Keep going

Regards

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strengthen - to make stronger
diminish - make or become less ; make something seem less valuable or impressive
propel - drive or push cause forwards
eschew - abstain from
belie - to give false impression of