Among the following, the element which show inert-pair effect are:

The correct options are
A $Bi$
B $Sn$
C $Pb$

The inertness of s subshell electrons towards the bond formation is called inert pair effect or it can be said as the inactiveness of electrons present in the outermost shell (i.e. $n{s}^2$) to get unpaired and involve in bond formation is called inert pair effect.

for example:

1) In 13th group, thallium can exhibit $+1$ and $+3$ oxidation states but it is stable in $+1$ oxidation state only due to inert pair effect

2) In 14th group, lead shows both $+2$ and $+4$ oxidation states but it is stable in $+2$ oxidation state due to inert pair effect.

3)In group 15 elements, ($Bi$) the stability of $+3$ oxidation state increases down the group and that of $+5$ oxidation state decreases down the group, due to inert pair effect.

Hence, the correct options are $A$, $B$ and $C$