In Frame Relay, the BECN 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 the Frame 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:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm