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).
Answer:
The author develops the marital harmony theme through plot details. The author develops the small-town gossip theme through setting details. The author develops the coming-of-age theme through point of view.
