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
+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.