Choice "A" is correct. In an assignment for consideration, the assignor and obligor are both liable to the assignee. However, if the assignee releases the obligor, it will serve to release the assignor as well. Thus, if Egan (the assignee) releases Barton (the obligor), West (the assignor) will be released as well.
Choice "b" is incorrect. In an assignment for consideration, the assignor and obligor are both liable to the assignee. Thus, if West (the assignor) released Barton (the obligor), West would remain liable to Egan (the assignee) because Egan paid consideration to be assigned West's rights.
Choice "c" is incorrect. In an assignment for consideration, the assignor owes a contractual duty to the assignee. If the assignor breaches his contract with the obligor so that the obligor is discharged from performing for the assignee, the assignee can hold the assignor liable.
Choice "d" is incorrect. In an assignment for consideration, the assignor and the obligor are both liable to the assignee. Thus, if Barton (the obligor) pays West (the assignor) it does nothing to extinguish the obligation owed to Egan (the assignee). Therefore, it would not constitute a release of West (the assignor).