"We are at a international conference" is wrong ('a' before a word that starts with a vowel just sounds wrong). It should be ""We are at AN international conference". The word 'an' isn't an option in the question.
The main answer, the one that you definitely should have been able to create, has "at the international". We do not accept "at a international", so if you found "a" among the tiles, it was one of the distractors (the wrong words).