Revenge was the main ( reason for - cause of - reason of - because of) the fire.
http://www.differencebetween.com/dif...and-vs-reason/
A cause is the one that produces the effect. For example you can say that a typical virus is the cause of the disease of chickenpox. It is understood that the particular virus produces the effect called chickenpox. Hence anything that produces an effect is called a cause.
On the other hand reason refers to a thought or a consideration in support of an opinion. Observe the usage of the word ‘reason’ in the sentences given below:
1. What is the reason for your decision?
2. You may give me the reason behind your suggestion.
In both sentences given above the word ‘reason’ is used in the sense of ‘a thought’ in support of an opinion or a decision.
cause تعبر عن سبب مادى زى شرارة النار ، أما reason تعبر عن دافع معنوى زى الحب أو الكره.