In the given sentence, the noun 'problem' is only known to the speaker, and can be any problem, it isn't specified. Hence an indefinite article, 'a' or 'an' is to be used. 'An' is used for non specific nouns which begin with a vowel and the noun 'problem' does not begin with a vowel. Hence option B is incorrect. 'A' is used for nouns beginning with a consonant, since 'problem' begins with the consonant 'p', option A is correct. 'The' is a definite article used for nouns that are specific, the noun being non-specific here makes option C incorrect. An article is not required before an uncountable noun since 'problem' is a countable noun, option D is incorrect.