For a message to be clear,it must:

A) define the purpose of the sender.
B) signal to the reader that it is a social message.
C) be organized so that form follows function.
D) use friendly language.

For a message to be clear, it should be organized in a way that its form follows its function. This means that the structure of the message should be determined by its purpose and intended audience, ensuring that the message is clear and understandable. Defining the purpose of the sender and using friendly language can also contribute to clarity, but they are not as essential as organizing the message effectively. Signal to the reader that it is a social message is not necessary to make a message clear.