Based on his purchases and payments, as well as the opening balance, Christopher's opening balance after December will be d. $513.74.
First find the balance on the card:
= Opening balance + Purchases - Payments
= 424.86 + 78.10 + 27.97 + 8.16 + 46.07 - 55.25 - 20
= $509.91
Opening balance becomes:
= 509.91 x (1 + 13.13%/365 days x 30 days)
= $513.74
In conclusion, option D is correct.
Find out more on credit card balances at https://brainly.com/question/6872962.