You should issue the clock rate command on the serial interface of Router B. Router B has been configured as data communications equipment (DCE), and the DCE device must provide clocking to establish the data communication speed for the link. A DCE device is the device that provides a clocking signal. A device that is not capable of providing or not configured to provide a clocking signal is considered the data terminal equipment (DTE) device. Although Cisco routers can be configured as DCE devices, they are typically considered DTE devices when connected to a Channel Service Unit (CSU)/Data Service Unit (DSU). You can verify whether a router has been configured as DTE or DCE by issuing the show controllers serial command.
When issuing the clock rate command, you should specify the parameter in bits per second. Thus, if you were to issue the clock rate 64000 command, you would configure the interface to operate at 64 Kbps. Router A has been configured as DTE. The DTE device accepts clocking parameters from the DCE device. Therefore, you should not issue the clock rate command on the serial interface of Router A.