Final answer:
The best definition of successful communication is that the listener understands the speaker's message as intended.
Step-by-step explanation:
The best definition of successful communication is (d) The listener understands the speaker's message as intended. This definition encompasses the ultimate goal of effective communication, which is for the listener to comprehend the speaker's message accurately and as it was meant to be understood. It implies that the speaker has conveyed their message clearly, and the listener has actively engaged and comprehended the message.