The correct answer is C. idioms.
Idioms are phrases which consist of many words which don't seem to be connected in any way, however, they create a new meaning which cannot be simply predicted - you just have to learn idioms by heart.
Imagery is using descriptive vocabulary to create vivid images which could be easily imagined; puns are play on words; malapropism is wrong usage of a word.
There is no dramatic irony in the pit and the pendulum but there is situational irony.
