The key here was to recognize that the action potentials displayed for you are those of nodal pacemaker cells -- NOT cardiac myocytes. Verapamil's effect on these cells is to slow depolarization, not decrease contractility (the cells have minimal contractile function). If you were shown myocyte action potentials, the answer would be decreased contractility.
I wish they used dihydropyridine drugs instead of non dihydropyridine drugs like verapamil/diltizem,that would be a much better question in my opinion.