What do you understand by $(a)$ inert pair effect,$(b)$ allotropy,and $(c)$ catenation?

(N/A) Inert pair effect: As one moves down the group,the tendency of $ns^{2}$ electrons to participate in chemical bonding decreases due to poor shielding by $d$- and $f$-electrons. This makes the lower oxidation state (group valency $- 2$) more stable.
$(b)$ Allotropy: It is the property of an element to exist in more than one physical form while maintaining the same chemical properties. These forms are called allotropes. For example,carbon exists as diamond,graphite,and fullerenes.
$(c)$ Catenation: It is the ability of atoms of an element to form strong covalent bonds with other atoms of the same element to create long chains or branched structures. This is most prominent in carbon.