Final answer:
The act that claimed Parliament's right to legislate for the colonies in "all cases whatsoever" was the Declaratory Act.
Step-by-step explanation:
The act that claimed Parliament's right to legislate for the colonies in "all cases whatsoever" was the Declaratory Act. In 1766, after repealing the Stamp Act, the new prime minister, Lord Rockingham, proposed the Declaratory Act to assert Parliament's supremacy and reaffirm its power over the American colonists. The act stated that any laws passed by the colonies that contradicted parliamentary law were null and void.