Which one of the following square planar complexes will form two geometrical isomers?

• A

  ${\mathrm{MA}}_4$

• B

  ${\mathrm{MA}}_3\mathrm{B}$

• C

  ${\mathrm{MAB}}_3$

• D

  ${\mathrm{MA}}_2{\mathrm{B}}_2$