Answer:
C. Library cards make it possible to read books all year long.
Step-by-step explanation:
The simplest and clearest language is in sentence C. Once you read it you know what it means. It says that If you have a library card, you can read books during the whole year. Then sentence A is too long and It is not clear at all. The same happens with sentence B. Then, sentence D is short but It is not clear at all because It doesn't say anything about the books or the library so It could be an application for any other thing.