Answer: D. Move the comma to after the word once.
Explanation:
There are many times a comma is necessary in a sentence and one such time is after the use of an introductory phrase. The introductory phrase here is, "Just this once", so there must be a comma after the word "once."
There is no need for a comma after the name of one of the people in the sentence because the conjunction, "and", is good enough when only two people are mentioned.
The comma should therefore be moved to after the word, "once".
