(b)
The higher the charge on the metal ion, smaller is the ionic size and more is the complex forming ability. Thus, the degree of complex formation decreases in the order
$M^{4 +} > MO_{2}^{2 +} > M^{3 +} > MO_{2}^{+}$
The higher tendency of complex formation of $MO_{2}^{2 +}$ as compared to $M^{3 +}$ is due to high concentration of charge on metal atom $M$ in $MO_{2}^{2 +}$