When would a military unit perform a 'Screen' operation?

A 'Screen' operation is specifically conducted to provide protection from enemy reconnaissance efforts. This type of operation involves the deployment of units in a way that allows them to observe and report on enemy movements while simultaneously ensuring the safety of their own forces. By doing so, a military unit is able to detect and disrupt enemy intelligence-gathering activities, thereby maintaining a tactical advantage. The primary goal of a screen is to deny the enemy the opportunity to gather information about friendly forces’ positions, dispositions, and intentions. This is crucial during the planning phases of operations as it helps to ensure the secrecy and effectiveness of subsequent maneuvers.

In contrast, advising higher command involves coordination and reporting on strategic decisions rather than direct engagement with the enemy. Conducting a major offensive would involve more aggressive and proactive actions rather than the defensive nature of a screen. Finally, while troop training exercises may involve elements of reconnaissance and movement, they are not classified as 'Screen' operations because their purpose is primarily educational rather than protective in nature. Therefore, the focus of a screen operation is primarily on countering enemy reconnaissance efforts, which confirms the correctness of the chosen answer.