Final answer:
The declarative solution to ensure a reason is entered when an opportunity is closed by a user is to have the system prompt the user to enter a reason, thus ensuring accountability and valuable data collection.
Step-by-step explanation:
The correct declarative solution for the scenario where a reason should be entered when an opportunity is closed by a user is: a) The system should prompt the user to enter a reason when closing an opportunity. This option ensures accountability and provides valuable data for the organization to understand the outcomes of opportunities. Options b, c, and d do not meet the requirement for a user to enter a reason when closing an opportunity and either prohibit the action or disregard the need for a reason.