A sulphide ore like $ZnS$ is first roasted to its oxide prior to its reduction by carbon because:

The correct option is C The Gibb's free energy for the formation of most sulphides are greater than that of $C{S}_2$
The Gibb's free energy for most of the sulphides are greater than that of $C{S}_2$. In fact, $C{S}_2$ is an endothermic compound. This means that the reduction of a metal sulphide by carbon to form metal and carbondisulphide is not favorable. Therefore, the ${\Delta }_r{G}^0$ of $M_{x}S$ is not compensated. So, the reduction of $M_{x}S$ is difficult. Hence it is a common practice to roast sulphide ores to their corresponding oxides prior to reduction.