What is the coordination geometry around the manganese atom in decacarbonyldimanganese$(0)$?

Square-planar geometry is shown by

The number of unpaired electrons in the complex $[CoF_6]^{3-}$ is ...... . (Atomic number of $Co = 27$)

An aqueous solution of metal ion $M1$ reacts separately with reagents $Q$ and $R$ in excess to give tetrahedral and square planar complexes,respectively. An aqueous solution of another metal ion $M2$ always forms tetrahedral complexes with these reagents. Aqueous solution of $M2$ on reaction with reagent $S$ gives a white precipitate which dissolves in excess of $S$. The reactions are summarized in the scheme given below:
$1.$ $M1$,$Q$ and $R$,respectively are :
$(A)$ $Zn^{2+}, KCN$ and $HCl$
$(B)$ $Ni^{2+}, HCl$ and $KCN$
$(C)$ $Cd^{2+}, KCN$ and $HCl$
$(D)$ $Co^{2+}, HCl$ and $KCN$
$2.$ Reagent $S$ is :
$(A)$ $K_4[Fe(CN)_6]$
$(B)$ $Na_2HPO_4$
$(C)$ $K_2CrO_4$
$(D)$ $KOH$
Give the answer for question $1$ and $2$.

Which of the following ions forms complexes with a coordination number of only $4$?

Explain on the basis of valence bond theory that $[Ni(CN)_4]^{2-}$ ion with square planar structure is diamagnetic and the $[NiCl_4]^{2-}$ ion with tetrahedral geometry is paramagnetic.