Answer:
a. debit
b. $90,000
Explanation:
The complete journal entries for the payment of principal at maturity is to debit  bond payable in the amount of $90,000, and credit cash  in the amount of $90,000 also. This will appear in the book as follows:
Details                   Dr ($)           Cr ($)
Bond payable            90,000
Cash                                   90,000
Being the bond payment of principal at maturity. Â Â Â Â Â Â
Therefore, if bonds are sold at par value, the issuer records the payment of principal at maturity with a debit to bond payable in the amount of $90,000.