The correct answer is A. Sender.
A sender is the person who initiates a communication by sending a message to a receiver. The sender is responsible for encoding the message into a form that can be transmitted to the receiver, and for selecting the appropriate channel for transmission.
The receiver is the person who receives the message from the sender. The receiver is responsible for decoding the message and interpreting its meaning.
A messenger is a person who delivers a message from one person to another. The messenger is not responsible for the content of the message, but only for its delivery.
A communicator is a person who engages in communication with another person or group of people. The communicator is responsible for both sending and receiving messages.
In the context of the question, the person who sends a message is known as the sender. The sender is responsible for encoding the message into a form that can be transmitted to the receiver, and for selecting the appropriate channel for transmission.