When you configure a T1 CAS PSTN Interface, which command instructs the router to receive its interface clocking from the service provider?

'linecode b8zs' configures the line coding format to B8ZS for T1, which ensures ones density on the circuit but has nothing to do with clock synchronization.

The 'clock source line' command under the controller T1 configuration tells the router to derive clocking from the physical T1 line delivered by the service provider. Since the PSTN is the authoritative clock source in the hierarchy, the customer equipment must slave to it to maintain synchronization and avoid transmission errors. This is the standard configuration for any customer-premises equipment terminating a provider-delivered T1.

'ds0-group 1 timeslots 1-24' defines a channel group mapping DS0s to a dial peer for CAS signaling and has no effect on clock source selection.

'clock source internal' configures the router to be the clock master using its own oscillator, which is incorrect when connected to a provider circuit that supplies the clock.