Question: What is the primary action taken by the mobile station to initiate service level up- or down-grading in the active state of a call?
Option 1: The mobile station sends a SETUP message with new bearer capability information.
Option 2: The mobile station sends a RELEASE message to release existing resources.
Option 3: The mobile station enters the "mobile originating modify" state without sending any messages.
Option 4: The mobile station sends a MODIFY message including the wanted value of specific parameters.
Explanation: According to the specified procedure, the mobile station initiates service level up- or down-grading by sending a MODIFY message that includes the desired values for parameters such as the "maximum number of traffic channels" and/or the "wanted air interface user rate". This action is taken while in the active state of the call.