Explanation:
Forward error control involves transmitting additional redundant information with each character or frame to facilitate detection and correction of errors, in feedback error control, only enough additional information is transmitted so the receiver can identify that an error has occurred.
Choices B and D are both error detection methods but not error correction methods. Block sum check is an extension of parity check wherein an additional set of parity bits is computed for a block of characters. A cyclic redundancy check is a technique wherein a single set of check digits is generated, based on the contents of the frame, for each frame transmitted.