The question supplies its own definition and then asks what follows from it, so the criterion must be read out of the definition rather than out of general knowledge about communication.

The definition speaks of transmitting a message from the sender to the other person. A message is not merely a sequence of words; it is the meaning and intent the sender puts into it. Transmission is therefore successful to the degree that the receiver reconstructs that same meaning and intent. That gives the fundamental yardstick: compare the meaning the sender intended with the meaning the receiver took, and the closer the match, the more effective the communication.

Noise level of the medium is a factor that can reduce fidelity, but it is a condition of the channel, not the measure of success. A noisy channel through which the intended meaning still arrives intact has produced effective communication.

Generic form of the content — whether the message is a lecture, a notice or a conversation — describes its format. The same intent can be carried effectively or ineffectively in any format, so form cannot be the criterion.

Simplicity of the message helps in many situations but is not the test either. A deliberately complex message that is understood exactly as intended is effective, while an oversimplified one that loses the sender's intent is not.

Hence the fundamental criterion is the meaning and intent as put in by the sender in communication.