Question: What happens when a mobile station clears an active call that is being pre-empted by another call with higher precedence?
Option 1: The mobile station sends a cause code indicating "call rejected" to the network.
Option 2: The mobile station sends a cause code indicating "user busy" to the network.
Option 3: The mobile station returns a cause code No. 25 "pre-emption" to the network.
Option 4: The mobile station sends a cause code indicating "call dropped" to the network.
Explanation: When a mobile station clears an active call that is being pre-empted by another call with higher precedence, it returns a cause code No. 25 "pre-emption" to the network, indicating that the call was cleared due to pre-emption by a higher priority call.