The scope of an operation is best described as:

The scope of an operation is best described as what needs to be done to reach the intended outcome because it encompasses the overall objectives and goals of the mission. This includes identifying tasks, assigning roles, and determining the necessary resources and strategies required for successful completion. In a rescue context, understanding the scope helps ensure that everyone involved is aligned with the mission's purpose and can work effectively towards a common goal.

The other choices, while relevant factors in a rope rescue operation, do not capture the full breadth of what "scope" entails. For instance, knowing the number of rescuers needed is essential for manpower planning, but it is just one aspect of a larger operational plan. Similarly, determining the length of rope required may be important for the specific mechanics of a rescue, but it does not address the operational objectives. Lastly, proper authority is crucial for safety and compliance, but it relates more to the leadership structure than to the operational plan itself. Therefore, defining scope focuses on the comprehensive picture of what actions and outcomes are essential for success.