Come on now THINK !!! The correct answer is indeed D. What caused the accident was stones on the road. You must look at "the cause" of the accident and what is "the cause"? It's not the stones - it's "stones on the road". "Stones on the road" is a condition, a single condition. We are not talking about many conditions on the road we are talking about one single condition.

It can't be as well "was stones" nor "were stone" The past form of verb to be " was" is associated with singular nouns ,while "were" with plural nouns. Those 2 examples attach "was" with a plural noun "stones" and " were" with a singular noun "stone". That is grammatically incorrect.

Hello RetSam, It is obviously "were stones". It could have been a stone but then the correct form would be " was a stone" , where the 'stone' has to be defined by a definite article "a".

b) were stones