Which of the following sets contain only isoelectronic ions? 

Ions are molecules with the same number of electrons as isoelectronic species, but with a different nuclear charge. 

${\mathrm{K}}^{+}=18$ electrons$(19-1)p^{3-}=18$  electrons $(15+3)$
${\mathrm{Ca}}^{2+}=18$ electrons$(20-2)s^{2-}=18$ electrons $(16+2)$
${\mathrm{Sc}}^{3+}=18$ electrons $(21-3){\mathrm{Cl}}^{-}=18$  electrons$(17+1)$
 ${\mathrm{Cl}}^{-}=18$electrons$(17+1)k^{+}=18$  electrons $(19-1)$

$\therefore$ The correct option is B and C.