Answer:
The medical assistant should identify that the child is using b. Regression as a defense mechanism.
Step-by-step explanation:
Regression is the defense mechanism in which an individual temporarily "goes back" to a previous stage in life in which he/she felt more secure.
We can see that the child presents this defense mechanism by sucking her thumb, which she hasn't done since she was a baby (now she is 5 years old). She is unable to deal with her current stage in life in an adaptive way and thus chooses to go back to a previous stage.