The correct option is
D who
A relative pronoun is used to connect a clause or phrase to a noun or pronoun. You see them used everyday with the most common relative pronouns being: who, whom, which, whoever, whomever, whichever, and that.
The sentence speaks about a person named Ashok. This person is usually appalled (horrified) by the war. So to dodge or avoid his fear, he has taken up Buddhism. From the explanation we can understand that this sentence is completely defining the person Ashok. Now, if we come to the part of the relative pronoun, we need something that should describe this person.
"who" is used to describe a person.
"whom" is used as an object of a verb or a preposition
"which" is used to describe a thing
"whose" is used as a possessive form of "who"
The best relative pronoun matching over here is "who" since it is describing the person, Ashok.
The answer for this question is option "C"- who