Usually represented as Ovals, but circles or rounded rectangles are fine.
Any shape that has a flat top side linked to a smaller flat lower side by equal, balanced, continuous (i.e. no breaks) sides will suffice. The prefferable shape is on the right side below.
Use a flat-based pentagon such as the one on the right below. A rounded top is also acceptable.
Virtually all literature available on decision theory depicts decision nodes as rectangles.
Hexagons are the preferred implementation, but parallelograms such as the one depicted below can be accepted