Answer:
Neutralization.
Explanation:
There are four techniques that are used to control the behavior of facial expressions. Such techniques consist of masking, intensification, neutralization, and deintensification, all more easily known/ referred to as MIND.
As given in the passage, Trish kept a straight face and pretended to not hear her best friend's revelation of the surprise birthday party for her. And in her expressionless reaction, Trish is exhibiting the neutralization technique of behavior. She is presenting an emotionless reaction, acting as if she knows nothing about the plan, hiding her true reaction, and showing a rather 'plain/ straight' face to her, eliminating any form or hint of her knowledge of the surprise.
