8.
The answer is: A
Explanation
The correct option is A: to, for.
The sentence "He is indebted to his friend for a large sum" uses the prepositions "to" and "for" correctly. Here's why:
"Indebted to" indicates the person to whom the debt is owed. In this case, he owes a debt to his friend.
"For" is used to specify the reason or cause of the debt. Here, it indicates the large sum that is the reason for the indebtedness.