The four words"weakness", "shortcoming", "demerit" and "defect" are four words shared with the similar meanings. In the dictionary, "weakness" and "shortcoming" have more to do with one's characer, while "demerit" and "defect" are used in a general sense. However, in this sentence, "The ___in David's character has hindered him from advancing in his character." Why the correct answer was said to be "defect" rather than the other three?
In what way was "defect" said to be the correct answer? Were the other three given as possible answers? Or were you specifically asked to fill in an answer without being given this list of options? If it was the latter, which answer did you give?
Any and all of these factors could be the reason that the answer given was 'wrong'...but if it is simply the word listed as 'the answer' in a book, it is also possible that the author of the question did not take synonyms into account, and only gave the one answer as a "correct" answer.
"Demerit" does not make quite as much sense, because a demerit is a factor given to someone, rather than inherent trait as the sentence suggests. And "shortcoming" would require some other context to clarify what shortcoming is being referred to, and if it were then the sentence would have to start with "This" as in "This shortcoming", but it does not. I could see "weakness" being correct, as well as "defect", but these subtle problems for the other two answers makes them lesser choices.
It's also possible you were asked to pick the "best" answer, which would mean the one that is accurate in most cases. In this case, "defect" would cover as broad a range of uses as possible, given no other context to the sentence.
Kind of arbitrary, there. Perhaps his defect is a tendency to redundancy, since one's character hindering one's character is a bit of a tautology.
Seriously, this seems like a test question, and the test is about prepositions, not words. Look at the hole - in David's...
The demerit in David's character has hindered him from advancing in his character.
-- Demerit is right out, because a demerit is based on actions, not nature, and character implies to nature.
The weakness in David's character has hindered him from advancing in his character.
The shortcoming in David's character has hindered him from advancing in his character.
-- Weakness and shortcoming need to be followed by 'of', not 'in' because they are non-specific and apply to the overall, not a specific aspect of the character. Therefore, they are of the character, not in it and are not correct here.
The defect in David's character has hindered him from advancing in his character.
-- Defect is a specific article, and therefore is part of the whole, and belongs with in.