Final answer:
Mark's insurance is the primary payer for Sally because his birthday comes first in the calendar year, according to the 'birthday rule' used in the insurance industry.
Step-by-step explanation:
When determining which insurance policy is the primary payer for Sally based on the birthday rule, we look at the birthdays of the parents, Mark and Grace, who both have family insurance benefits. The primary insurance policy is determined not by the age of the parents, but by whose birthday (month and day, not year) occurs first in the calendar year. Since Mark's birthday is on July 10, 1968, and Grace's birthday is on September 5, 1967, Mark's insurance is considered the primary policy because his birthday comes first in the year. Therefore, the correct answer is c. Mark's insurance, as his birthday comes first in the calendar year.