Solution:(b) Rajat first heard the story of his uncle's mysterious disappearance in his childhood.(Correct)(a) The story uncle heard first time was about Rajat's mysterious disappearance in his childhood. (Incorrect Sentence Structure)
(c) The story Rajat first heard in his childhood was about his uncle's mysterious disappearance. (Incorreot Sentence Structure)
(d) Rajat's first story that uncle heard in his childhood was about his mysterious disappearance. (Incorrect Sentence Structure)