Which one is the softest solid non-metal among the following? 

Bromine is present as a liquid or gas. The hardest naturally occurring substance is diamond.
Sulfur is a yellow solid that has a rotten egg odour. It is a weak electrical conductor and brittle solid. At room temperature, bromine is a crimson liquid. The softest solid non-metal from them is, sulphur. As oxygen and bromine are not solids, being soft or hard does not apply to them.

Hence, the softest solid non-metal among the given options is 'Sulphur'.