Read the given sentence and choose the correct modal auxiliaries to fill in the blanks.
Granny _____ sew without glasses.

The correct answer is: Mustn’t

Option a is eliminated as it does not make sense. Option c is not appropriate either as "cannot" means "is not able to"- but Granny might be able to sew without new glasses. So her ability isn't in question.
Thus the correct answer is option b- "mustn't", which means that even though Granny can, she should not as it could be harmful for her.