Answer: D) Octagon; not regular
Step-by-step explanation:
There are 8 sides total since there are 2 sides per "arm" of this 4-sided star shape. So we have an octagon.
This octagon is not regular because some of the internal angles are over 180 degrees. This is due to this polygon being concave. In a concave polygon, some vertices are closer to the center compared to other vertices. For a regular polygon to be possible, all vertices must be the same distance away from the center.
In the diagram below, I have marked the vertices in red and blue. The red vertices are closer to the green center point, compared to the blue vertices.