"shut your trap" is an example of

a. a simile
b. an idiom
c. onomatopoeia
d. personification

The correct answer is B. An idiom

Explanation:

The word idiom refers to a word, phrase or group of words that are colloquially used in everyday and informal conversations and whose meaning cannot be understood from the literal meaning or the meaning of each individual word. This implies idioms message is not literary expressed and due to this, idioms can only understand from a figurative meaning or a non-literal meaning. In the case of "Shut your trap" this is a colloquial form to ask someone to stop talking, whose meaning is quite similar to the phrase "shut your mouth", however, instead of the word "mouth" the word "trap" is used to replace it and thus, the meaning of this phrase depends on the figurative meaning instead of the literal meaning as "trap" usually refers to an object or plan to trap something or someone, but in this case is being used to refer to mouth, thus, as these words are being used for colloquial contexts and their meaning rely on the figurative language rather than in the literal language "shut your trap" can be classified as an idiom.