The quote by Maximilien de Robespierre, "Terror is nothing other than justice, prompt, severe, inflexible," best applies to the Committee of Public Safety under Robespierre's leadership in the sense that they believed in using severe and inflexible measures in order to ensure justice.
The Committee of Public Safety was established during the French Revolution and its primary goal was to protect the revolution and maintain order. Under Robespierre's leadership, the committee implemented policies that were aimed at eliminating perceived threats to the revolution. They believed that in order to achieve justice and maintain the revolution, it was necessary to use swift and severe actions against those who were considered enemies or counter-revolutionaries.
Therefore, the quote suggests that the Committee of Public Safety was willing to use extreme measures, including the use of terror, to ensure justice and preserve the revolution. This means that the Committee of Public Safety forcefully eliminated all opposition in their quest to establish and protect the ideals of the revolution.