Quote Originally Posted by Crowe Diddly View Post
Forget about the sentence as a whole. The only thing that matters is the modifying clause "that everyone IS posting about". In that clause, 'everyone' is the subject. 'Everyone' is a singular pronoun. It gets IS, not ARE.

Answer is 100% A.
This is oversimplification.

...Take this example and consider in the sentence in question that there are two subjects: The dog is being annoying. The cats are being annoying. Everyone is being annoying. -- combined: The dog, the cats, and everyone are being annoying. Do you not see how if you changed ARE to IS it would leave out the other nouns? Are the gif things not being posted about?