Final answer:
The operationalization in the pen-in-mouth experiment refers to the act of putting a pen in your lips.
Step-by-step explanation:
The operationalization in the pen-in-mouth experiment refers to the act of putting a pen in your lips. Operationalization is the process of turning abstract concepts into measurable variables or actions that can be observed or measured. In this experiment, the pen-in-mouth action serves as an operationalization because it represents a specific behavior that can be objectively observed and measured to draw conclusions about the relationship between facial expressions and mood or attitudes.