答案：Which of you can think of a situation （in）which this idiom is often used?
Which of you can think of a situation （which） this idiom is often used （in）?
Which of you can think of a situation （where） this idiom is often used?
当先行词是situation,scene,等时,定于从句中用：where/in which或者末尾有in时,前面加which/that/不填.