What defines a "fix" tactical mission task?

A "fix" tactical mission task is defined by the objective of preventing the enemy from moving for a specified period. This task is crucial in military operations because it allows friendly forces to consolidate their strength and either prepare for an offensive action or support another operation. By fixing the enemy in place, a force can disrupt their plans, control the tempo of the battle, and create opportunities for other units to maneuver.

In this context, the option regarding keeping an enemy from moving aligns perfectly with the intent of a fix mission. It emphasizes the importance of maintaining pressure on the enemy, ensuring they are unable to reposition or reinforce their units during key phases of combat.

The other options describe different tactical actions but do not capture the essence of a fix task. Preventing friendly units from retreating focuses more on friendly force discipline. Attacking the enemy's rear lines involves a distinct offensive action rather than the restraining purpose of a fix. Finally, providing reconnaissance for forward units is related to gathering intelligence, which is critical for planning and executing operations, but it doesn't encompass the specific task of immobilizing the enemy. Thus, the true nature of a "fix" tactical mission task lies in its purpose to halt enemy movement to facilitate the success of broader military objectives.