Which monarch rejected the colonies' conciliatory measure and declared the colonies to be in rebellion?

King George III is the monarch who rejected the colonies' conciliatory measure and declared the colonies to be in rebellion. His response came during a critical moment in the lead-up to the American Revolution when tensions were escalating between the American colonies and British authorities.

By refusing to accept the colonies' attempts to resolve their grievances peacefully, King George III not only demonstrated his commitment to maintaining British authority over the colonies but also solidified the perception among colonists that they had no viable option for reconciling their differences with the British government. His declaration essentially marked the turning point that fueled colonial resistance and ultimately contributed to the outbreak of war.

In contrast, the other monarchs listed did not play a role during this period or in relation to the American colonies' rebellion. King George II reigned prior to King George III and had less direct influence on events leading to the American Revolution. King James I's reign ended in the early 17th century, well before the colonial tensions emerged, and King Charles I was executed in 1649, long before the fight for American independence began. Thus, King George III stands out as the critical figure in this historical context.