What does one do if, a party at trial denies having made a statement in a published article?
A combination of a) and b).
The plaintiff should gather other evidence with which to disprove the denials of authorship, or at least to question the reliability the publisher's testimony. The jury needs that evidence in order to discern who is credible. Whether decisive or not for credibility purposes, any material inconsistencies the plaintiff is able to point out from the witness's testimony(-ies) tend to guide the fact-finder (i.e., the jury) on matters of credibility.
Whenever possible, the evidence should include admissible documents from when the events took place. For instance, records reflecting the submission & editing process of that article. Those documents typically are obtained by subpoenaing the publisher and/or relevant non-parties, accordingly. That will reduce the chances of testimony deficiencies, whether these stem from witnesses' perjury, lack of memory, inability to clearly articulate what they know, or inability to testify at all (due to witness's illness or death by the time the matter goes to trial).