What is the significance of keying the mic on a certain frequency 3/5/7 times?

The correct answer pertains to signaling for emergency assistance. Keying the microphone a specific number of times, such as three, five, or seven, is a common practice in aviation communication protocols. This method serves as a distress signal to indicate that an aircraft is experiencing an emergency situation. Each number of key presses can have particular meanings or urgency levels in some systems, which is crucial for ground control and other aircraft to recognize that immediate attention or assistance is needed.

The other options do not fulfill this role. Changing a flight plan typically requires a formal request and coordination through air traffic control, and it wouldn’t be signaled simply by keying the mic. Adjusting cabin temperature is managed through the aircraft's environmental control systems rather than through radio communications. Similarly, controlling lighting intensity is also not a function of keying a mic; instead, it is accomplished through the aircraft operations management systems. Thus, signaling for emergency assistance is the primary significance of keying the mic on a specific frequency multiple times.