Yes, A is wrong because ‘most’ means greater than 50%, whereas ‘often’ just means greater than one. This means that we know that some mystery stories feature a brilliant detective who solves the mystery presented in the story, but we have no idea whether or not this happens in more than half of all mystery stories.
Similar to ‘often’ (and ‘many’), ‘some’ simply means more than one. So, if mystery stories often give readers enough clues to infer the correct solution to the mystery, this means that some mystery stories give readers enough clues to infer the correct solution to the mystery. This is why C is correct.
While we’re on the topic: The LSAT likes to trick people into making bad inferences when they see the word ‘many’. ‘Many’ means more than one, but it doesn’t necessarily mean more than 50%. So make sure to distinguish between ‘many’ (more than one, but we have no idea how many) and ‘most’ (more than 50%).