A. Sales
B. Purchases
C. Inward returns
D. Closing stock
The closing stock D. is the value of goods which remain unsold at the end of the period whose balance appears once in Trading Account and once in Balance Sheet of the business.
All other accounts sales A., purchases B. and Inward Returns C. are closed once they are absorbed by the Trading Account. Thus D. is the correct answer.
The correct answer to the question: "The balance of which of the following accounts do not disappear, once they are debited/credited to Trading Account?" is "Closing stock".