What is the primary characteristic of an ASCII control character?

The primary characteristic of an ASCII control character is that it performs a functional action without displaying a character. ASCII control characters are part of the ASCII standard and are designed to control the flow of data in text processing. They include characters such as line breaks, carriage returns, tabulators, and other non-printable characters that serve specific functions in data transmission and formatting, rather than representing visible symbols or characters.

In contrast, the other choices focus on characteristics that are not associated with control characters. While printing characters on the screen is relevant to standard ASCII characters, it does not apply to control characters, which are not intended for display purposes. The option regarding converting letters to numbers pertains more to data encoding and character representation, rather than the function of control characters. Lastly, the encryption of data for security involves algorithms and processes that are distinct from the role of ASCII control characters, highlighting a different area of computing altogether.