Here it says which of the following represents a weak binary acid and therefore a weak electrolyte. So remember, a binary acid does not possess oxygen, so automatically all the ones with oxygen are out.
So we have HCL and HCN. So they both represent binary assets because of the lack of oxygen. But the one that's a weak electrolyte would have to be the one that is also a weak acid.
HCL is one of our strong acids, so it would be a strong electrolyte. So by default, HCN, which is hydrocyanic acid, would be our weak binary acid and therefore our weak electrolyte.