Explanation
WPA2 (WiFi Protected Access II) is a WiFi security option that uses encryption and authentication to protect the wireless network from unauthorized access or eavesdropping. WPA2 is the most secure and recommended WiFi security option among the options given. If some light switches are no longer communicating after replacing the wireless router with a new device that uses WPA2, the most likely cause is that the light switches do not support WPA2. The light switches may need to be updated or replaced to be compatible with WPA2.
The router operating on a different channel, the key not meeting password complexity requirements, and the SSID not being broadcast are not likely causes of the light switches not communicating after replacing the wireless router with a new device that uses WPA2. References: CompTIA IT Fundamentals+ Study Guide:
Exam FC0-U61, Second Edition, Chapter 8: Security Concepts, page 311.