Final answer:
Mark is the primary payer because the court decree specifies that he is responsible for Sally's healthcare expenses.
Step-by-step explanation:
The primary payer in this situation would be Mark because the court decree indicates that he is responsible for Sally's healthcare expenses, regardless of custody arrangements. The court decree is a legally binding document that outlines the responsibilities of each parent, and in this case, Mark has been designated as the responsible party for healthcare expenses.