The listens to disclosure.
a. channel
b. message
c. receiver
d. source

The correct answer is c. receiver.  The receiver listens to disclosure.

In this sentence we are presented with a communicative situation and the options are the elements of this situation. In a normal communicative situation we would have a source that wants to transmit some information to another person. The source then emits a codified message carying that information through a communicative channel that finally reaches the receiver, who then interprets the message.

In the mentioned sentence, the verb "listens" is describing the action performed by the receiver of the message, who is listening to the disclosure.