Final answer:
3. Maximilien Robespierre cut off the king's head by the guillotine during the Reign of Terror.
Step-by-step explanation:
The person who cut the king's head off by the guillotine because they were afraid he would commit treason by helping France's enemies was 3. Maximilien Robespierre.
Louis XVI, the king of France, was found guilty and executed on January 21, 1793 at the Place de la Concorde. Marie Antoinette, his queen, was also executed in October of 1793. These executions took place during the Reign of Terror, a period of extreme violence and political repression in France.