JEE Main 26 Feb 2021 Shift 2 Solved Paper

Calgon is used to remove permanent hardness of water by removing cation Mg2+.
Calgon is not an element, it is a compound. The 2 nd most abundant element by weight in the Earth's crust is silicon (Si). So, the statement in option (a) is not true.
Calgon is sodium hexametaphosphate (NaPO3)6 or Na2[Na4(PO3)6]. It is a polymeric water soluble compound. It is also named as Graham's salt.
Mg2+ of hard water is removed as

Na2[Na4(PO4)3]

Calgon

+2Mg2+(aq)⟶Na2[Mg2(PO3)6](s) +4Na+(aq)
Ca2+ ion cannot be removed by calgon as Ca2+ is larger in size than Mg2+.
So, the statement of option (b), (c) and (d) are true.