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 workflow rule with the followingformula: AND( ISCHANGED(StageName), NOT(ISCHANGED(NextStep))
B . Create a validation rule with the following formula: AND( ISCHANGED(StageName), NOT(ISCHANGED(NextStep))
C . Create a validation rule with the following formula: AND(ISCHANGED(ISPICKVAL(StageName)), NOT(ISCHANGED(NextStep)))
D . Create a workflow rule with the following formula: AND( ISCHANGED(ISPICKVAL(StageName)), NOT(ISCHANGED(NextStep))

Answer: B