Final answer:
Option (b), The execution of King Louis XVI marked a shift to a radical phase in the French Revolution by symbolizing a break from the old regime and creating a power vacuum that led to the Reign of Terror.
Step-by-step explanation:
The execution of King Louis XVI pushed the French Revolution into a radical new phase because it symbolized a break from the old regime. His death marked the irrevocable collapse of absolute monarchy and led to the establishment of a republic, thereby changing the political landscape of France. This execution revealed the depth of the revolutionaries’ commitment to dramatically restructure society, leading to the radical period of the Revolution known as the Reign of Terror, which involved widespread violence and political purges.
The execution created a power vacuum, as evidenced by the chaos that ensued afterward, and it marked the true point of no return from monarchical rule to a new form of governance. This reflective of the Jacobin ascendancy and the radicalization of the revolution, a shift from moderate reformist phases to a more extreme pursuit of republican principles. It also had significant international repercussions, such as altering the view of the revolution abroad, with some seeing it as a step too far towards anarchy.