Why are sulphide ores usually converted into oxides before the extraction of metals?

The correct option is "Metal oxides are easier to reduce than sulphides".

In metallurgy, the extraction of a metal from its concentrated ore typically involves reducing the metal compound to its elemental form. Reduction is the process of gaining electrons or removing oxygen/electronegative elements.

Sulphide ores are chemically very stable because the bond between the metal and sulfur is quite strong. Reducing a metal sulphide directly using common reducing agents like carbon (coke) or carbon monoxide is thermodynamically unfavorable and requires extremely high temperatures, which is not economically viable. The Gibbs free energy change ($\Delta G^{\circ }$) for the reduction of sulphides by carbon is positive or insufficiently negative under standard metallurgical temperatures.

On the other hand, metal oxides can be reduced much more easily. Reducing agents like carbon, carbon monoxide, or hydrogen have a high affinity for oxygen and can readily reduce metal oxides to the respective metals at relatively lower temperatures. For example, carbon reacts with oxygen to form stable carbon dioxide or carbon monoxide, making the overall reduction reaction thermodynamically favorable with a highly negative Gibbs free energy change.

Therefore, sulphide ores are first subjected to roasting (heating strongly in the presence of excess air) to convert them into metal oxides:
Metal Sulphide + Oxygen ⇒ Metal Oxide + Sulfur Dioxide
Once converted, the resulting metal oxide is easily reduced to the free metal using a reducing agent.