The correct option is
B b
The first statement in the sequence should be the one that introduces a new topic. It should not be a continuation or extension of a topic.
d is first in the sequence: d is the only phrase that starts a new topic. It tells us that the children requested Ramesh to tell them stories. Since, all the other phrases talk about these stories, d will come first.
d is followed by c: c is the continuation of d. It tells us that when the children requested Ramesh to tell stories, he conceded to their request.
c is followed by a: a tells us what happened after Ramesh conceded to the children's' request. It says that the children sat listening to his stories for almost an hour.
a is followed by b: b adds details to the topic discussed in a. It says that the stories being told by Ramesh were full of crime and mystery. Thus, b follows a.
Thus, b is the fourth in sequence. Hence, B is the correct option.