Which of the following is an example of a system Electronic Protection (EP)?

The correct choice is associated with a key aspect of Electronic Protection (EP). Passive detection is primarily focused on the ability to gather and analyze incoming signals without actively transmitting signals that could give away the operator's position. In the context of Electronic Warfare, EP entails measures that protect against electronic attacks and enhance the survivability and effectiveness of systems. Passive detection serves as a protective measure by allowing systems to remain stealthy and reduce the likelihood of detection or targeting by an adversary.

In contrast, the other options typically relate to other concepts within the broader field of Electronic Warfare. For example, pulse compression is more about enhancing the resolution of radar systems, and pulse coding usually pertains to modulation techniques that improve communication reliability and resistance to interference. Frequency agility refers to changing frequencies to avoid detection and jamming but does not inherently align with the protective measures of EP in the same way that passive detection does. Thus, the context of passive detection—utilizing stealth and situational awareness—makes it a fitting example of Electronic Protection.