Final answer:
Displacement is the defense mechanism where a person transfers their emotions from the original source to a different, less threatening target, such as a nurse in this scenario.
Step-by-step explanation:
The client becoming angry and belligerent toward the nurse after speaking on the phone with his mother is exhibiting the defense mechanism known as displacement.
Displacement involves transferring inappropriate urges or behaviors onto a more acceptable or less threatening target. Instead of expressing his frustration towards his mother, with whom he might have had an upsetting conversation, the client redirects those feelings towards the nurse, who is a less threatening object of those aggressive feelings.