You have to find unit cells of two-dimensional space groups (or the bases of parallelepipeds of three-dimensional groups)

The base of a unit cell can be as small as possible a square, rectangle, rhombus, or parallelogram. It is often possible to select another cell with the same area (two-dimensional).

Search your crystallography textbook for plane symmetry group (s) or Wikipedia for Wallpaper group (very complex explanation).