Which of the following complexes is capable of showing optical isomerism?

Total number of geometrical isomers of $[Ma_3b_3]$ is/are:

Explain geometrical isomerism in six-coordinated complex compounds or octahedral complexes.

Which of the following can show optical isomerism?

Which of the following statements is true?

The $d-$ electron configurations of $Cr^{2+}, Mn^{2+}, Fe^{2+}$ and $Co^{2+}$ are $d^4, d^5, d^6$ and $d^7$ respectively. Which one of the following will exhibit the lowest paramagnetic behaviour? (Atomic no. $Cr = 24, Mn = 25, Fe = 26, Co = 27$).