Final answer:
Under the birthday rule, Patty's insurance is the primary payer for Monica, because Patty's birthday comes earlier in the calendar year than Bill's.
Step-by-step explanation:
The question asks which parent's insurance policy will be the primary payer for Monica under the birthday rule, which is a common method used by insurance companies to coordinate benefits when a child has coverage under both parents. According to the birthday rule, the policy of the parent whose birthday falls earlier in the calendar year (month and day, not year) is considered the primary insurance. Since Monica's mother, Patty, has a birthday of March 15, 1968, and her father, Bill, has a birthday of September 25, 1967, Patty's insurance would be the primary payer because her birthday comes first in the year.