1. Could you please explain why 'b' isn't correct?
Is this item from some Advanced Mastery of English Comprehension test?
If the children are 'settling down/have had time to settle down', then the teacher has already got their attention away from their own activities and directed them (back) to her next learning activity, so she now needs them to focus their attention on reading some story in a book.
2. Note that the given sentence is: Put your _____ into reading!
The expression we have is, "put your heart into it/your work/something".
If somebody say, "put your mind into it', it sounds like they are accusing them about going about some job or activity in a careless, mindless way.
Also, the request being made is not to be boring when you read. That is, put some life into your reading, some excitement and passion. We see that as 'emotion', and in our culture, the heart is the seat/source of emotion, not the mind.
Student or Learner