Choice "D" is correct. In the absence of an agreement to the contrary, the assignment of a contract does not relieve the assignor of his/her obligations under the contract. Here, Ace remains liable to Wilcox even though Ace assigned (delegated) its contractual responsibility to Pure.
Choice "c" is incorrect. Notice alone isn't enough to remove liability. Ace could avoid liability only by an agreement among all of the parties substituting Pure for Ace and releasing Ace. Such an agreement is known as a novation.
Choice "a" is incorrect. Only duties that involve specialized personal skills (i.e., rely heavily on the personal attributes of the person performing) cannot be delegated, and a contract to paint a warehouse does not rely on specialized skills, as would a contract to paint a portrait.
Choice "b" is incorrect. The duty to paint a building does not involve highly specialized skills and, therefore, is delegable. Since Ace's duty to paint Wilcox's warehouse could be delegated, the act of delegating was not a breach of contract.