How can this process be automated?

The VP of Sales wants to require that the Next Step field is always updated when an opportunity stage is changed.

How can this process be automated?
A . Create a validation rule with the following formula: AND( ISCHANGED(ISPICKVAL(StageName)), NOT ( ISCHANGED(NextStep)))
B . Create a validation rule with the following formula: AND( ISCHANGED(StageName), NOT ( ISCHANGED(NextStep)))
C . Create a workflow rule with the following formula: AND( ISCHANGED(StageName), NOT ( ISCHANGED(NextStep))
D . Create a workflow rule with the following formula: AND( ISCHANGED(ISPICKVAL(StageName), ISCHANGED(NextStep))

Answer: A