In describing the elements of communication, a person or an event that provides a verbal or non-verbal cue to which someone can respond is known as the

A person or event that gives a verbal or non-verbal cue to which someone can respond is the source.

The source originates the communication by offering the first cue that starts the exchange.

The message is the content that is sent, not the origin that provides the cue.

The channel is the medium carrying the message, distinct from the one who initiates it.

The communication context is the setting surrounding the act, not the originator of the cue.

As the starting point of the process, the source sets the whole exchange in motion.