B.
Both my daughters regressed. In fact, we're still working with the second one right now. What I found helped with both is just to take away the diapers. It will be messy for the first few days but since they know what to do already they will pick it back up quickly. You may want to wait a few weeks since you have a newborn or do it now and take advantage of the baby taking a lot of naps still. I hope whatever you choose to do works out well and quickly! I know how fustrating it is.