Which of the following is not optically active?

Which of the following statements is not correct?

What kind of isomerism exists between $[Cr(H_2O)_6]Cl_3$ and $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$?

Which of the following complexes will show coordination isomerism?
$A. [Ag(NH_3)_2][Ag(CN)_2]$
$B. [Co(NH_3)_6][Cr(CN)_6]$
$C. [Co(NH_3)_6][Co(CN)_6]$
$D. [Fe(NH_3)_6][Co(CN)_6]$
$E. [Co(NH_3)_6][Fe(CN)_6]$
Choose the correct answer from the options given below:

Maximum number of stereoisomers are possible for the octahedral complex $Mabcdef$ :-

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. $Cr = 24, Mn = 25, Fe = 26, Co = 27$).