Indicate the complex ion which shows geometrical isomerism-

1. ${\left[\mathrm{Cr}{\left({\mathrm{H}}_{2}\mathrm{O}\right)}_{4}{\mathrm{Cl}}_{2}\right]}^{+}$

2. $\mathrm{Pt}{\left({\mathrm{NH}}_{3}\right)}_{3}\mathrm{Cl}$

3. ${\left[\mathrm{Co}{\left({\mathrm{NH}}_{3}\right)}_{6}\right]}^{3+}$

4. ${\left[\mathrm{Co}{\left(\mathrm{CN}\right)}_{6}\right]}^{3-}$

Hint: General formula [M(A)4(B)2] show geometrical isomerism

${\left[Cr{\left({H}_{2}O\right)}_{4}C{l}_{2}\right]}^{+}$shows geometrical isomerism because it is a MAB, type coordination

compound which contains two set of equivalent ligands, four HO and 2 Cl.

Hence, the possible geometrical isomers are

Hence, correct choice is (a).