What position would be used to repel an enemy attack to your flank?

The correct answer is supplementary position. A supplementary position is established to cover tactical maneuvers and support primary or secondary positions during combat scenarios. Its purpose is to offer additional firepower or support in situations where the enemy may attempt to attack from the flank or surreptitiously move around primary defenses.

In the context of a flanking attack, the supplementary position would allow friendly forces to effectively engage and deter the enemy while maintaining the integrity of their primary defense. This strategic use ensures that if the enemy maneuvers to exploit vulnerabilities, friendly forces can provide immediate support and counteract the threat, helping to prevent breaches in their defensive lines.

Each of the other options has specific roles: a primary position serves as the main defensive setup, a secondary position acts as an alternate if the primary is compromised, and a fallback position is a predetermined location to regroup if a withdrawal is necessary. However, a supplementary position is specifically intended to enhance coverage and protection against flanking assaults, thus making it the appropriate choice in this scenario.