The correct order of the conversation is: B. a-b-d-c-e

This order follows a natural flow of conversation:

John introduces his plan: "I'm planning to visit Japan next summer." (a)
Mary asks a question: "Why Japan?" (b)
John explains his reason: "I've always been fascinated by Japanese culture and history." (d)
Mary asks a follow-up question: "Have you already decided on the places you want to visit?" (c)
Mary expresses her enthusiasm: "That sounds amazing! I hope you enjoy the trip." (e)