British experience in the Boer War has shown that "D" is the correct answer. A battalion has 900 riflemen. A trained rifleman can fire twenty aimed shots per minute. That means a battalion can fire 18,000 aimed rounds in a minute. No enemy attacker can stand up to that firepower for long.

Machine guns are unreliable. They can jam or overheat easily, and they are not accurate. They simply spray rounds wildly. Aimed rifle fire is much more effective

Artillery bombardments can be effective but communications between front line troops and the artillery batteries is slow. An enemy could cross the entire battlefield before the artillery is ready. Attackers also move quickly so the gunners would have to continually readjust their aim.

Aircraft are slow, flimsy and can barely carry even a pilot, let alone weapons. By the time an aircraft warmed up, took off, and reached the battlefield, the attack would be long over.