Sulphide ion is soft base and its ores are common for metals
(a) Pb (b) Al (c) Ag (d) Mg
Choose the correct answer from the options given below

According to the Hard-Soft Acid-Base (HSAB) principle, soft bases prefer to combine with soft acids, and hard bases prefer hard acids. The sulphide ion ($$\text{S}^{2-}$$) is a soft base because of its large size and high polarisability.

Among the given metals: Lead ($$\text{Pb}^{2+}$$) is a soft/borderline acid — its common ore is galena ($$\text{PbS}$$). Silver ($$\text{Ag}^+$$) is a soft acid — its common ore is argentite ($$\text{Ag}_2\text{S}$$). Aluminium ($$\text{Al}^{3+}$$) is a hard acid with a small, highly charged ion — it forms oxide/hydroxide ores such as bauxite ($$\text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O}$$). Magnesium ($$\text{Mg}^{2+}$$) is also a hard acid — it occurs as magnesite ($$\text{MgCO}_3$$) and dolomite, not as a sulphide ore.

Since sulphide ores are common for the soft-acid metals Pb and Ag, the correct combination is (a) and (c).

The answer is option (1).