The term diction simply refers to an author's choice of words. Diction errors involve words that are incorrect in a particular context, either because they have the wrong meaning or because they do not follow the conventions of standard written English.
English contains many fixed phrases, known as idioms. Idioms are not correct or incorrect for any logical reason; they simply reflect the fact that certain phrases have evolved to become standard usage. As a result, there is essentially no way to study for these types of questions other than to read. English contains far too many idioms to memorize, and there is no way to predict which ones will appear on any given test.
Home Work: pages 73-75