Which of the following elements has variable valency?

Correct option is A

The correct answer is (a) Iron.
 Iron exhibits variable valency, meaning it can form compounds in multiple oxidation states. The most common valencies of iron are +2 (ferrous) and +3 (ferric).
Information Booster:
 Sodium (Na):
o Valency: Sodium typically exhibits a valency of +1.
o Usage: It is highly reactive and is used in various compounds like sodium chloride (table salt).
 Sulphur (S):
o Valency: Sulphur can exhibit multiple valencies, commonly -2, +4, and +6.
o Usage: It is used in the production of sulfuric acid and in vulcanization of rubber.
 Magnesium (Mg):
o Valency: Magnesium typically exhibits a valency of +2.
o Usage: It is used in the production of lightweight alloys and as a reducing agent in the extraction of metals.