A _____ equals a frequency of one trillion cycles in one second.

The correct answer is terahertz, which signifies a frequency of one trillion cycles per second. The term is derived from the metric prefix "tera," denoting a factor of (10^{12}) or one trillion. Thus, when measuring frequencies using the terahertz unit, it represents substantial speeds typically associated with advanced technologies, such as high-speed data transmission in telecommunications and optical communications.

In contrast, the other terms reflect different scales of frequency. Gigahertz refers to (10^9) cycles per second (one billion cycles), megahertz stands for (10^6) cycles (one million cycles), and kilohertz represents (10^3) cycles (one thousand cycles). Therefore, terahertz is the only unit among them that corresponds to a trillion cycles per second, making it essential for understanding high-frequency applications in modern technology.