Question: In which of the following scenarios does the "Active" state (U10) exist in a mobile network?
Option 1: When the MS is idle and not engaged in any calls.
Option 2: When the MS is in the process of setting up a call, but has not yet received an answer from the remote user.
Option 3: When the MS has sent a call setup request, but has not yet received a response from the network.
Option 4: When the MS has answered a mobile terminating call or has received an indication that the remote user has answered a mobile originating call.
Explanation: The "Active" state (U10) exists when the MS has established a connection with the remote user, either by answering a mobile terminating call or receiving confirmation that the remote user has answered a mobile originating call. This indicates an active communication session between the MS and the remote user.