Which mission task is characterized by moving indirectly against an enemy while still maintaining a positioning advantage?

The task characterized by moving indirectly against an enemy while maintaining a positioning advantage is indeed "Screen." In military operations, screening involves using forces to observe and report enemy movements while providing a protective barrier for other units. This allows for the preservation of tactical advantage, as the screening force can engage or disengage from the enemy at will, enabling the main body to maneuver freely without direct confrontation.

This indirect engagement and ability to maintain positioning allows the screening unit to gather intelligence and shape the enemy's decision-making without fully committing to a confrontation. The nature of this mission task emphasizes the importance of observation and flexibility, which are critical in maintaining the initiative in military operations.

In contrast, other tasks listed like securing or supporting by fire tend to involve more direct confrontation or engagement with the enemy, rather than the maneuvering and positioning characteristic of screening operations. Thus, "Screen" is the most fitting description of the task presented in the question.