Gee, hmm. Is 'George' female or male? Shouldn't it be 'his sister'?
 George travelled 500 miles to get to hospital to visit his sister, Mary, . . . was thoughtful of him.
a. that b. which c. who d. what
Here's how to work out the correct answer for . First of all, you're looking for a relative pronoun.
Question: What was thoughtful of George?
Answer: He travelled to visit his sister, Mary. (clause)
The underlined clause, (c), 'who was thoughtful of him' means Mary cared about George. (d) 'what' doesn't function as a relative pronoun in that context. (a) 'that' doesn't function as a relative pronoun in that context, and
(b) 'which' does indeed function as a relative pronoun.
Is the correct answer (c) or (b)? What does the sentence express? It it about George or Mary? For example, if it's about George travelling a great distance to see someone he cares about, then (c) isn't the correct answer.