What caused the accident was/were the stones on the road.

• .1.参考答案-1
  不严格的讲,都对.
  主语从句通常谓语动词用单数.但在口语中,可以根据表语的数来推断主语从句所指的是单数还是复数.
  所以,非要选一个的话,就选were.
  但并非一楼所说的倒装句,而是通过表语判断主语的数,主语还是what引导的从句
• .2.参考答案-2
  was
  因为What caused the accident 是主语，名词性从句谓语动词用单数。
• .3.参考答案-3
  were stones 加了s了 ，所以是were 还有这个句子是倒装句
• .4.参考答案-4
  was
  it was the stones on the road that caused the accident。