The given answer is incorrect. 'bangles' is plural and if the indefinite article is required it should be in the plural, 'some'.
No. Bangili could be singular or plural and "bangle" is not a plurale tantum in English. "A bangle" is completely correct, but yes, it should also accept:
I agree with the five answers you give. However the given answer in the exercise is "A bangles", notwithstanding the correct answer given in the discussion above - 'A bangle'. To use the singular indefinite article with the plural form of the noun is simply incorrect.