What do we call a person who is unable to pay his/her debt?
A. Bankrupt
B. Borrower
C. Loanee
D. Obliged

Hint: Debt is a noun which means owing someone money to someone that was previously borrowed.
Complete answer:
From the hint given to us, we know that debt is a noun which means owing someone money to someone that was previously borrowed. Since the question asks for a name, the word must be a noun.
Bankrupt is used for a person who is unable to repay his/her debts. We observe that the meaning of bankrupt matches the description in the given sentence. Hence, option A is possibly the correct answer.
Borrower is the person who has borrowed an entity from someone. We observe that the meaning of the borrower does not match the description in the given sentence. Hence, option B is not the correct answer.
Loanee is a person who takes the loan. We observe that the meaning of the borrower does not match the description in the given sentence. Hence, option C is not the correct answer.
Obliged means to be grateful. It is an adjective. Neither does it match the description in the given sentence nor is it a noun. Hence, option D is not the correct answer.
Thus, we can conclude that option A is the correct answer.
Note: Bankrupt is also used as an adjective and a verb. However, since in the given sentence it used to define a certain person, it is a noun.