The question is a multiple choice
I don't think you have any real cause _____ complaint.
Choices are ( of / for / with / on)
Clearly it's either of or for. In my model answer it should be "for"
"Revenge was the main _____ the fire.
Choices ( reason for / cause of / reason of )
It should be "reason for".
There must be a difference between cause and reason in their usages and also prepositions ?