The correct option is D why; antecedent - reason
A relative pronoun is one that relates to a preceding noun or pronoun called the antecedent, and connects the clause which it introduces to the antecedent. The noun is "reason" and it precedes the relative pronoun "why" introducing the clause, "why I have called you". Hence, Option D is
correct. The rest of the options do not adhere to this rule, hence
incorrect.