For (a) I think the only symmetry group that works is the identity because any reflection or rotation that is not by $360°$ changes the figure, for (b) I believe it's $D_4$ as rotations by $90°$ and reflections leave the figure untouched and for (c) I think it's $D_3$ for the same reason (but in this case the rotations are by $120°$). Is my reasoning correct?