Final answer:
The National Convention of 1792's first major step was the abolition of the monarchy, making the transition to a republic, rather than the Declaration of the Rights of Man, the execution of the king, or the creation of the Committee of Public Safety.
Step-by-step explanation:
The first major step taken by the National Convention of 1792 was the abolition of the monarchy, which is option D. This significant act marked the transition from a constitutional monarchy to a republic in France. Prior to this, the Legislative Assembly had suspended King Louis XVI and charged him with treason, leading to his eventual execution. While the Declaration of the Rights of Man and of the Citizen, the execution of King Louis XVI, and the formation of the Committee of Public Safety were important events during the French Revolution, they were not the National Convention's first step.
The National Convention, composed of various revolutionary groups with differing opinions on the government's future, ultimately aligned with the Jacobin's radical faction known as the Mountain faction after a trial, resulting in the decision to execute King Louis XVI in January 1793. The earlier reforms and actions, such as the Declaration of the Rights of Man, influenced by Enlightenment ideals and supporting natural rights, equality before the law, and freedom of expression, set the stage for the Convention's bold first step.