Currently there are 25 native Standard Prime lenses for Pentax K20D and 18 of these lenses are autofocus (AF) type.
K20D also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on K20D.
Since Pentax K20D has environmental sealings (Weather Sealing), you may like to couple your camera with one of these 3 Standard Prime lenses with Weather Sealings.