Correct answer: A. It helped detect German aircraft in time
Explanation: Correct answer: A. Radar is a system that sends out radio waves and detects their return from distant objects, such as aircraft. During the Battle of Britain in 1940, Britain used radar stations to receive early warning about approaching German aircraft. This gave the Royal Air Force time to prepare pilots, organize fighter aircraft, and direct air defence toward the danger. Option B is wrong because radar detects objects; it does not manufacture atomic bombs. Option C is wrong because digging trenches is a human military engineering task, not the function of radar. Option D is wrong because radar did not itself defeat Japan, and the Battle of Britain concerned the defence of Britain against Germany. Memory cue: radar means “early warning,” not a weapon that directly destroys the enemy.