Pretty sure it's C. while that's what i'd go with, it could be B.
Cats make good house pets. It's the main idea in the sentence, and it's independent also. The other clause is dependent, so it is not the main clause.
What is the prompt of the essay?
Essentially a thesis statement should state your opinion/the main idea of your essay. If a thesis is a question, it is no longer a statement.
Depending on what you will be writing about here are some alternatives that are close to your original thesis:
Social media has negative/positive effects on young adults. (discuss positive or negative effects)
Social media can affect young adults in a variety of ways. (discuss both positive and negative effects)
Young adults are affected by social media in many positive/negative ways. (discuss positive or negative effects)
