A requirement for a hotel reservation case type states that as customers change their room...

To meet the requirement that the total cost of a hotel reservation updates as customers change their room selection, using a declare expression is the most effective approach.

D. Define a declare expression for the total cost field: A declare expression allows for the automatic calculation of the total cost field whenever related properties, such as room selection, change. This setup ensures that the total cost reflects the current room selection dynamically, providing an updated cost to the customer without manual intervention.

Configuring a Validate rule (A) is typically used for verifying that entered data meets certain criteria, not for dynamically updating values. Applying a Disable when condition (B) would not contribute to the calculation or update of the total cost but would instead control the editability of the field. Adding a When condition (C) to the room selection step could influence the flow of the case but does not directly result in updating the total cost.