The answer would be A, Price.
Lucy is an entrepreneur who started a chocolate (Product) business. She has found a local candy store (Place) that will sell bars and they will also be available online. She has also planned to purchase ad space in a local magazine (Promotion).
Now she has done with all the variables of marketing mix except Price. She has decided the place, she has come up with the promotional strategy, She has the product with her. So the only thing that needs to be decided is the Price.