The argument will be strengthen by the fact that the advertisers choose a decision based on moral grounds even though they would be stand to loose financially on withdrawing from the publication.

Advertisers will be acting morally and not financially from withdrawing. And this strengthens the reasoning as it wants to conclude of showing how advertisers behave financially and morally AS WELL.

A is partially true; but C is clearly true. Because it fully and explicitly talks about the main intention of the withdrawal!
It says that advertisers change the publication they give advertisements, not for monetary issue but for moral reasons.

A is the second best, but is not complete, since it does not give full answer to the intention of the withdrawal...

C.