Which of the following compounds has a zero magnetic moment?

For an octahedral complex $MX_{4}Y_{2}$ ($M=$ a transition metal,$X$ and $Y$ are monodentate achiral ligands),the correct statement among the following is

Which of the following complex compounds exhibits geometrical isomerism?

Total number of unpaired electrons in the complex ions $[Co(NH_3)_6]^{3+}$ and $[NiCl_4]^{2-}$ is...............

The electronic configurations of $Cr^{2+}, Mn^{2+}, Fe^{2+},$ and $Co^{2+}$ are $d^4, d^5, d^6,$ and $d^7$ respectively. Which of the following has the lowest magnetic moment? $[Cr = 24, Mn = 25, Fe = 26, Co = 27]$

How many geometrical isomers are possible in the following coordination entities?
$(i)$ $[Cr(C_2O_4)_3]^{3-}$
$(ii)$ $[Co(NH_3)_3Cl_3]$