A a question word, referring to the termidiom.
B a conjuction, linking the main sentence“An idiom is a phrase or expression” to the subordinate clause “cannot be understood”(...).
C a relative pronoun, being used to indicate that the following noun belongs to or is associated with the person or thing mentioned in the previous clause.
D a conjuction, linking the main sentence “An idiom is a phrase or expression” to the coordinate clause“used to convey a figurative or metaphorical meaning”.