Well, one would have to read the article to answer, however, I think it's B
It is False that an observer must know that social learning is occurring for it to be effective.
The second choice is a fragment, not a complete sentence.
I believe the correct choice is number one, because if you turn the sentence around, it answers the question of why the crowd is gathered. You may want to confirm on the website chompchomp.com (Grammar Bites).
Flowing water
Hope I Helped :)
