The newspaper concludes that unions are declining in strength. The evidence put forward by the newspaper is the declining number and size of strikes. The author takes this same piece of evidence and twists it around to imply the denial of the conclusion. The author uses the fact that there are fewer and smaller strikes to suggest that the unions are not weaker and may in fact be stronger. This is perfectly summarized in answer choice (C).
(A) is not true. The author doesn't question the evidence, just the conclusion reached from it.
(B) states something that never happened. It's not that the evidence the newspaper used is outdated, rather, the author states that it leads to a different conclusion.
(C) is correct for the reasons above.
(D) states something that never happened. The author does not question the motivations of the newspaper article's conclusion.
(E) mentions common interests between unions and management. The author mentions common interests between unions and others. Not exactly the same thing!