Final answer:
To ask US leads for their state on a form, the marketer should use visibility rules.
Step-by-step explanation:
The functionality that the marketer should use to ask US leads for their state on a form is Visibility rules. With visibility rules, the marketer can create conditional logic that shows or hides certain fields based on specific conditions. In this case, the condition would be the lead's country. If the lead selects the United States as their country, the state field will be visible for them to enter their state.