This seems more like an opinionated question, and less of one that has a real answer. However, I would choose B because it would make a follow up question easier, and it seems like it’s more of what the teacher would be looking for.
Answer:
For fear of retaliation against them.
Explanation:
Douglas wouldn't want fot the boys who taught him how to read to get in trouble on his behalf. He was afraid that they would be repremended or punished for having taught him how to read.