Which of the following can show optical isomerism?

Which of the following is not correctly matched?

Number of possible isomers for $[Cr(H_2O)_2(C_2O_4)_2]^{-}$ are . . . . . . .

The spin-only magnetic moment value of $M^{n+}$ ion formed among $Ni$, $Zn$, $Mn$, and $Cu$ that has the least enthalpy of atomisation is $........$ (in nearest integer). Here $n$ is equal to the number of diamagnetic complexes among $K_2[NiCl_4]$, $[Zn(H_2O)_6]Cl_2$, $K_3[Mn(CN)_6]$, and $[Cu(PPh_3)_3I]$.

Which one of the following has an optical isomer?
$(en = \text{ethylenediamine})$

Which one of the following complexes will most likely absorb visible light? (At. nos. $Sc = 21$,$Ti = 22$,$V = 23$,$Zn = 30$)