Premul said to his sister, ''Will you go to school? ''

Reported speech -এ Auxiliary verb 'will' থাকাতে that না বসে if/whether বসাতে হবে।
Structure: Subject + ask/inquire of + object (যদি থাকে) + if/whether + reported speech এর subject + verb + বাকী অংশ৷
অর্থাৎ, সূত্রানুযায়ী সঠিক উত্তর- Premul asked his sister if she would go to school.