The radio communications system that assigns numbers to common phrases is called _____.

The correct answer is that the radio communications system that assigns numbers to common phrases is known as Ten-codes. These codes, also referred to as 10 signals, were developed to help streamline communication and enhance clarity among emergency responders. By using a set of standardized codes instead of full phrases, Ten-codes minimize the chance for miscommunication, especially in high-pressure situations where quick, clear communication is crucial.

For instance, when emergency personnel use codes such as "10-4" to indicate acknowledgment or agreement, it helps convey information rapidly and efficiently while also reducing the amount of transmission time needed. This is particularly important in emergency response scenarios where time is of the essence.

The other options, while related to communication systems, do not specifically refer to the standardized numeric codes used to convey common phrases in the same way. Alpha-numeric codes can be a means of communication but do not align precisely with the structured format of Ten-codes. Radio jargon refers more to informal language or slang used among radio operators rather than a formal coding system. Pro-Code does not represent a widely recognized term within the context of radio communications used by emergency services.