(c)
On the basis of precipitation concept an electrolyte will be precipitated firstly whose ionic product is exceeded to its solubility product. In similar ionic conc. of cations, an electrolyte is precipitated firstly for which $K_{sp}$ is lower. In these electrolytes $MnS,\ FeS,\ ZnS\ \ and\ HgS,\ HgS$ is precipitated firstly.