The given clause, "whoever wins the contest a savings bond" is a subordinate or a dependent clause. This kind of clause does not convey a complete thought and needs the main sentence in order to make it understandable. This subordinate clause is connected to the main clause with the subordinating conjunction "whoever".
Okay IF I REMEMBER CORRECTLY I believe option C because you see the three witches prophesied to him saying he would be throned king !!!
