What primary metric of scanning can stations use to select the best AP for connectivity...

When a station scans for available wireless networks, it listens for beacon frames sent by APs. A beacon frame contains information about the BSS, such as SSID, supported rates, channel, security, etc. The station also measures the signal strength of the beacon frames, which indicates how well the station can communicate with the AP. The signal strength is usually expressed in dBm or RSSI units. The higher the signal strength, the better the connection quality and performance. Therefore, the station can use the signal strength of AP beacons as the primary metric to select the best AP for connectivity to the desired BSS12. References: CWNA-109 Study Guide, Chapter 6: Wireless LAN Devices and Topologies, page 249; CWNA-109 Study Guide, Chapter 6: Wireless LAN Devices and Topologies, page 243.