The most appropriate thesis sentence would be: "As we got older our lives went in different directions, but we still keep up on social media" (D).
In a good thesis sentence, your reader should:
identify a key question or idea,
be able to understand how you are going to develop this idea in the paper,
and be given a hint of what your conclusion will be.
This sentence (D) is raising a key question: how did the friendship evolve as you got older? It is also providing a direction regarding the way you are going to approach the development (chronologically). Finally, it mentions the conclusion, or the answer to the question, in a few words: "we still keep up on social media."
