How is the overall commander decided when there are two units making up a fixing force?

The correct reasoning for determining that the larger unit's commander leads when two units make up a fixing force is based on the principle of unity of command and efficiency in military operations. Leadership in a joint effort typically favors the command structure that can provide the most resources and coordination. The commander of the larger unit usually has more assets at their disposal, which can enhance situational awareness and control over the battlefield. This consolidation of command helps to maintain clear lines of authority and reduces ambiguity in decision-making, which is crucial during complex operations.

In practice, leveraging the capabilities of the larger unit allows for a more cohesive strategy against the opposing forces, ensuring that both units work effectively towards a common objective under a single leadership framework. Essentially, by placing the larger unit's commander in charge, it streamlines communication and reinforces coordination, vital components for success in any military operation.