A pilot flame detector can include which of the following components?

In the context of pilot flame detectors, a flame rod is a crucial component that directly senses the presence of a flame. The flame rod works by detecting the ions produced in the flame, which creates a conductive path. When the flame is present, current can flow through the rod, signaling that the burner is operational. This functionality is essential for the safety and efficiency of combustion systems, as it ensures that the fuel is only supplied when there is a flame present to ignite it.

Other components like solenoids, regulators, and pressure gauges serve different purposes in the system. A solenoid is typically used for mechanical actuation, controlling the flow of gas but does not directly detect flames. Regulators are used to control the pressure of gas entering the system, and pressure gauges monitor gas pressure, neither of which contributes to flame detection. Thus, the flame rod stands out as the component specifically designed for detecting pilot flames, making it the correct answer in this context.