In Frame Relay, the FECN bit is set by:

A.
The Frame Relay Network, to inform the DTE receiving the frame that congestion was experienced in the path from source to destination

B.
The Frame Relay network, in frames traveling in the opposite direction from those frames that encountered congestion

C.
The receiving DTE, to inform Relay network that it is overloaded and that the switch should throttle back

D.
The Sending DTE, to inform the Frame Relay network that it is overloaded and that the switch should throttle back

E.
Any device that uses an extended DLCI address

Explanation:
The FECN bit is part of the Address field in the Frame Relay frame header. The FECN mechanism is initiated when a DTE device sends Frame Relay frames into the network. If the network is congested, DCE devices (switches) set the value of the frames’ FECN bit to 1. When the frames reach the destination DTE device, the Address field (with the FECN bit set) indicates that the frame experienced congestion in the path from source to destination. The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow control may be initiated, or the indication may be ignored.