What term indicates that a transmission has been received and understood?

Answer: C — Roger

The term "Roger" is widely recognized in aviation and radio communication as a signal indicating that a transmission has been received and understood. This term stems from the phonetic alphabet used in radio communication, where "Roger" represents the letter "R," which stands for "received." When a pilot or air traffic controller responds with "Roger," it confirms that they have comprehended the message conveyed to them. In contrast, while the other terms such as "Affirmative," "Ten-four," and "Copy that" may also convey acknowledgment, they are used in different contexts or have varying levels of formality. "Affirmative" is more formal and is often used in military or structured communications; "Ten-four" originates from citizens band radio (CB) communication; and "Copy that" is common in communications but does not carry the same standardized usage in aviation contexts as "Roger." Thus, "Roger" stands out as the preferred term for indicating a message has been received and understood in air traffic communication.