Which of the following has the largest number of isomers?

Which of the following does not show optical isomerism?
$(en = \text{ethylenediamine})$

The spin-only magnetic moment value for the complex $[Co(CN)_6]^{4-}$ is $...... \ BM$ [At. no. of $Co=27$]

Which of the following complexes has a magnetic moment of $2.82 \ B.M.$?

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ The spin only magnetic moment value for $[Fe(CN)_6]^{3-}$ is $1.74 \ BM$,whereas for $[Fe(H_2O)_6]^{3+}$ is $5.92 \ BM$.
Reason $R:$ In both complexes,$Fe$ is present in $+3$ oxidation state.
In the light of the above statements,choose the correct answer from the options given below:

Which of the following compounds cannot show coordination isomerism?