When people are unsure of what to say, they tend to decrease their communication effectiveness by speaking softly. This example best illustrates in nonverbal communication.
a. proxemics
b. kinesics
c. paralanguage
d. metalanguage

c) para language

Explanation:

Para-language is the language tat is used for conveying message or communication of emotion or modifying meaning through intonation technique, pic factor, volume factor , prosody etc.It is also known as vocalic.

According to the question,para language is used when a person tend to decrease  their volume or tone of speaking through soft speaking as to decrease effectiveness of communication.

Other options are incorrect because example of kinetics, meta-language and proxemics is not displayed through case.Thus, the correct option is option(c).

are there any options?

answer; deindustrialization led to move disposal facilities; (b);