The sulphide ores of metals, which lie in the middle of activity series, are first converted to oxides. The necessary condition for this conversion is:

Correct option is C

The correct answer is (c) presence of excess air

Explanation:
• Crucially, many metals strategically located right in the middle of the established chemical reactivity series (such as common zinc, iron, and lead) predominantly exist in raw nature as tightly bound sulphides or carbonates.
• In industrial metallurgy, it is chemically and economically much easier to successfully extract a pure, elemental metal from its oxide form than directly extracting it from its complex sulphide or carbonate form.
• Therefore, the highly crucial process of aggressively and strongly heating sulphide ores specifically in the constant presence of excess air is scientifically and industrially known as Roasting.
• A classic, foundational chemical textbook example is the roasting of zinc blende ore: $2ZnS + 3O_{2} \rightarrow 2ZnO + 2SO_{2}$.
• This absolutely essential conversion step effectively transforms the stubborn sulphide into a far more manageable metal oxide, which can subsequently be easily reduced to yield the pure elemental metal.

Information Booster:
• Conversely, the remarkably similar process of actively heating carbonate ores very strongly in strictly limited air to effectively convert them into oxides is formally called Calcination.
• Following either process, the subsequent reduction of these resulting newly formed metal oxides to completely pure metals is typically and cheaply achieved using powerful reducing agents like Carbon (coke).

Additional Knowledge:
presence of high temperature (Option a)
• While achieving a high temperature is indeed practically used during the process, the unique and distinguishing "necessary condition" for actively converting sulphides specifically is the mandatory chemical presence of flowing oxygen.

presence of high pressure (Option b)
• Applying physically high pressure is absolutely not a primary or necessary environmental factor utilized anywhere in this specific metallurgical roasting process.

absence of oxygen (Option d)
• Deliberately heating in the complete absence or severely limited supply of ambient oxygen precisely describes Calcination, which is strictly applied to carbonate ores, absolutely not sulphide ores.

So the correct answer is (c)