The correct answer is C. Substituting
Explanation:
Non-verbal language or behavior relies on gestures, movements, and other visual elements and non-verbal elements to communicate a message. Additionally, this type of language can have multiple functions such as reinforcing, contradicting or substituting. In the case of substituting, this occurs when non-verbal language is used to replace one or multiple words. This can be seen in the case presented because the signal made by Libby's friend replace multiple words such as "Remain quiet" or "Please keep quiet" and therefore the function of this behavior is substituting.
