Select non-metals other than carbon which shows allotropy.

Allotropy is the phenomenon of the occurrence of an element in different forms having different physical properties. 

Non-metals are substances that do not have metallic bonding. In the given options mercury and potassium are metals. The remaining sulfur and phosphorous have different forms. 

The allotropes of sulfur are orthorhombic S, Monoclinic S, and plastic sulfur. 

The allotropes of phosphorous include white P, red P, and black P. thus, the non-metals that show allotropy other than carbon are sulfur and phosphorous.
Hence, option A and B is correct.