Which tactical operation involves gaining time to protect the main body?

The tactical operation that involves gaining time to protect the main body is the guard. In military terminology, a guard is a defensive operation that primarily focuses on delaying the enemy, allowing the main body of forces to prepare or reposition themselves as necessary. This operation is crucial in situations where the main force needs to maintain strength or regroup, and the delaying action provides the required time for that to happen.

In a guard action, the forces are usually deployed in a manner that ensures they can engage the enemy effectively while still maintaining the ability to withdraw or retreat if the situation demands it. This tactic enables the main body to execute its objectives without immediate threat while the guard unit absorbs and mitigates the enemy's initial contact.

The other options describe different tactical concepts. Envelopment typically involves maneuvering around an enemy force to attack its flanks or rear, which is more focused on achieving a decisive blow rather than delaying. Infiltration suggests covertly moving personnel through enemy lines to reach a strategic position, emphasizing stealth and surprise rather than protection. Movement to contact denotes a maneuvering tactic aimed at finding the enemy and establishing contact, actively seeking engagement rather than delaying for protection.