What action by the British government prompted Americans to design the mock stamp on the cartoon?

The action by the British government that prompted Americans to design the mock stamp was the implementation of the Stamp Act in 1765. This legislation required colonists to pay a tax on printed materials, including newspapers and legal documents, effectively putting a financial burden on them. The mock stamp in the cartoon served as a form of protest, symbolizing the colonists' resentment towards taxation without representation and their demand for rights.