Answer :
The correct answer is: "Congress can override or cancel a President's veto; the bill then becomes a law".
The Unites States federal organization has been designed according to the principle of separation of powers. Therefore, there are three independent branches of state power: legislative (Congress), executive (federal goverment) and judiciary (Court system).
Moreover, a system of Checks and Balances has been established to guarantee that none of the branches gathers enough power to overrule the others, and each branch is granted powers to supervise and limit the actions undertaken by the other two. For example, the Congress can override the veto of the President if certain majority voting requirements are met. Another example could be the power of judicial review exercised by the court system, which can abolish certain actions undertaken by the legislative or executive powers if those are considered unconstitutional.