OEThe
“While” and the hypothetical
“if what he said were true” in the first part of the sentence indicate that he is actually a liar. This is based on his long track record, so a good fill-in might be an established liar. Libelous is a trap answer—libel is lying in print for the purpose of damaging someone’s reputation. Libelous liar would actually be redundant—and, of course, you have no indication that the lying was done in print. Nullified (invalidated, voided), unverified (unconfirmed), and forfeited (lost as a result of crime or fault) all don’t quite work as a description of a liar; to the extent that they do, they cast doubt on his ability to lie, a doubt this sentence doesn’t support. Inveterate (long-established and unlikely to change) is the correct answer.