True, though I interpreted the meaning here as "(pairs of) trousers" and not just one pair. Having said that "a cheap pair of trousers is not comfortable" would carry the same meaning I guess. Did the programme suggest "a pair of trousers are" as a valid answer? If it did, report it.
In the end, as long as the colloquial terms are not being translated differently from the less informal term, both options should be available, even though it might sound colloquial in the alternate translation. This course is primarily directed towards teaching a language in general, not to be finally able to converse with attendants of a black-tie assemblies.
'trousers' is a plurale tantum, that is, a word that exist as plural only. Rules of English grammar say that you shouldn't use indefinite article in front of plural nouns.