Answer:
The answer is reaction formation.
Step-by-step explanation:
Reaction formation is a defense mechanism characterised by showing extremely opposite feelings to one's real emotions, often in an exaggerated fashion. In the example, the mother may know it's not socially acceptable to show hostily towards her child, so she presents a radically different behaviour to coneceal her true feelings.
This mechanism is believed to be caused by a person's fear of someone discovering the truth about their ideas. This means they will take any possible chance to show the opposite attitude.