What is the recommended approach to meet this requirement?

Universal containers would like to ensure that when an opportunity stage is closed lost the reason is captured in a custom reason lost field before the record can be saved.

What is the recommended approach to meet this requirement?
A . Create a trigger that requires reason lost to be populated once the opportunity stage is closed lost.
B . Create a workflow rule that fires on the closed lost stage and populates the reason lost field.
C . Create a page layout for closed lost opportunities and make reason lost a required field.
D . Create a validation rule that requires reason lost to be populated once the opportunity stage is closed lost.

Answer: D