Under which rule is a child's primary insurance coverage under the father's plan when both parents have insurance?

The correct answer pertains to the concept commonly known as the birthday rule. This rule is a guideline used by health insurance companies to determine which parent's insurance plan is considered primary for covering a child's healthcare expenses when both parents have insurance.

Under the birthday rule, the insurance plan of the parent whose birthday falls first in the calendar year is designated as the primary insurance for the child. This approach is intended to provide a standardized method for determining insurance coverage without delving into the specifics of custody arrangements or gender considerations. In practical terms, this means if one parent's birthday is in January and the other’s is in December, the parent with the January birthday would have their insurance plan cover the child’s medical expenses first.

Options regarding gender or custody do not accurately reflect the method used for determining primary insurance coverage. The gender rule would imply a method based on which parent is male or female, which is not a standard practice in insurance coverage determination. The parent rule and custody rule do not align with common practices either, as they might suggest coverage based on living arrangements or other subjective factors rather than the straightforward and objective birthday rule.