_____ wins this civil war there will be little rejoicing at the victory.

The correct answer is option -B 

Whoever- It is a subject pronoun & work like the pronouns I,We He,She & They.

Ex- Whoever wins this debut will win a prize.

Whoever means -Someone/anyone who does a particular thing or is in a particular situation.

Meaning of the remaining options

1) Whatever- Unexceptional /unimportant,at all, absolutely, whatsoever.No matter which,for any, anything that.

Synonyms- So what.

Ex- Whatever may happen,I will go to Mahabaleshwar today.

2) Wherever-(Adverb) An emphatic form of where.

It also acts as a conjunction -whatever place, anywhere,in all places, everywhere.

Ex- I will come with you wherever you will take me.

3) Whichever- (Adjective / pronoun) - Any one/ a no of a group.

Ex- Whichever pizza you ordered for her, it must have had some very delicious ingredients.