“Single image” force loading a single file. If you have the volume in a list of PNGs then you must keep “Single image” option unchecked.
By the way, where these PNGs come from? It would be much better to save the volume properly, as a 3D nrrd or mha file, instead of storing as a list of 2D files (which has many problems, for example there is no standard way of storing 3D position, orientation, and spacing of slices).