Mr$.$ A has bought a new $90-$ HP out$-$board motor. He told his friend, Mr$.$ B that he will give him his old $35-$HP motor as soon as the new one arrives. Mr$.$ B has a small boat, but this small boat cannot accommodate the old engine. Mr$.$ B needs to modify his small boat, but he will have to spent expensive modification. With Mr$.$ A$\text{'}$s suggestion, he bought a new boat. When the new $90-$HP motor arrived, Mr$.$ A changed his mind and decided to sell the old one to other party. Is there a binding contract between Mr$.$ A & Mr$.$ B$?$

a$.$ Yes, because gratuitous promise is enforceable under the law of contract.

b$.$ No$,$ because there was no consideration from Mr$.$ B$.$

c$.$ No$,$ because, according to strict rules of common law, gratuitous promise remains gratuitous.

d$.$ Yes, under the doctrine of equitable estoppel, a promisor by his$/$her conduct induced promisee to rely on his promise and the promisee did rely upon it$,$ to his$/$her injury.