Answer: C. A base, by definition, reacts with an acid to neutralise it — producing a salt and (usually) water. Every base does this; it is the defining property. Option C is correct.
A is wrong: bases include metal oxides (\(\text{Na}_2\text{O}\), \(\text{CaO}\), \(\text{CuO}\)) and metal hydroxides. Non-metal oxides are usually acidic, not basic.
B is wrong: only the soluble bases (called alkalis) form alkaline solutions. Insoluble bases like \(\text{CuO}\) and \(\text{Fe(OH)}_3\) do not dissolve and so don't form alkaline solutions.
D is wrong: ammonium salts are formed when an acid reacts with ammonia (the base \(\text{NH}_3\)), not when every base reacts with ammonia in general. It describes one specific base, not a property all bases share.
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