The correct answer is 'at' because we use 'at' to refer to specific points in time, in this case "at the start of the year". We use in for longer periods of time like months or years (in June, in 2012, etc.). On is used for specific days or dates (on Monday, on 9th June, etc.). By means "no later than" so we can't use that in this case. You could say something like "I want to finish reading this book by January".