Choice "D" is correct. Essentially, by the assignment, Burns sold the contract right to collect the $800 from Nagel. In such a case, the assignor warrants that he does not know of anything that would impair the value of the assignment; otherwise, people would attempt to assign contracts whenever they knew of a problem.
Choice "b" is incorrect. There is no implied warranty that the promisor will perform.
Choice "a" is incorrect. The obligor need not be given notice to effectively assign a contract right. However, until the obligor receives notice, no liability is incurred by paying the assignor.
Choice "c" is incorrect. An assignee generally is subject to all of the defenses that the promisor would have against the assignor relating to the contract (e.g., that the car was stolen).