$Cis-trans$ isomerism is found in square planar complexes of the molecular formula: ($A$ and $B$ are monodentate ligands):

In an experiment,$m$ grams of a compound $X$ (gas/liquid/solid) taken in a container is loaded in a balance as shown in figure $I$ below. In the presence of a magnetic field,the pan with $X$ is either deflected upwards (figure $II$),or deflected downwards (figure $III$),depending on the compound $X$. Identify the correct statement$(s)$:
$(A)$ If $X$ is $H_2O_{(l)}$,deflection of the pan is upwards.
$(B)$ If $X$ is $K_4[Fe(CN)_6]_{(s)}$,deflection of the pan is upwards.
$(C)$ If $X$ is $O_{2(g)}$,deflection of the pan is downwards.
$(D)$ If $X$ is $C_6H_{6(l)}$,deflection of the pan is downwards.

In a $d^4$ low spin complex,what will be the magnetic moment $(\mu)$ in $B.M.$ if the coordination number is $6$?

In a particular isomer of $[Co(NH_3)_4Cl_2]^0$,the $Cl-Co-Cl$ angle is $90^\circ$. This isomer is known as:

Which of the following complexes or complex ions can have an optically active isomer?

Which of the following will exhibit optical activity?