Diana works as the manager of an office in a large corporation. she promotes one of her best workers, helen, in order to groom her for a future management position. in this example, which style of leadership would be most effective to meet diana’s goals?

Good Morning!  The answer would be delegative.  

In delegative style of leadership, the leader allow its employees/ team members to make their own decision.

In order to prepare Helen for future management position, Diana has to train Helen to make her own decision so she'll be eligible for the position in the future.  Hope this helps! :-)