Context clues are hints that an author gives to help define a difficult or unusual word within a book. The clue may appear within the same sentence as the word to which it refers or it may follow in the next sentence. Because most of our vocabulary is gained through reading, it is important that we are able to recognize and take advantage of context clues.
a, b, and c are all correct.
this is b. how railways dealt with people riding the trains without paying