What is the coordination number of $Fe$ in $[Fe(CN)_6]^{3-}$?

Identify the total number of complexes having bidentate ligands in them from the following list of complexes:
$a$) Tetracyanonickelate$(II)$ ion
$b$) Trioxalatocobaltate$(III)$ ion
$c$) Sodium hexafluoroaluminate$(III)$
$d$) bis(ethylenediamine)dithiocyanatoplatinum$(IV)$

Consider the following statements according to Werner's theory:
$1.$ Ligands are connected to the central metal ions by coordinate bonds.
$2.$ Secondary valencies have directional properties.
$3.$ Secondary valencies are non-ionisable.
Which of these statements are correct?

Order of the number of chelate rings is correct for:
$(i). \, [Co(en)_2(Ox)]^+$
$(ii). \, [Fe(Trien)(Ox)]^+$
$(iii). \, [Ca(EDTA)]^{2-}$
$(iv). \, [Pt(Ox)_2(H_2O)_2]$

The complex $[Pt(NH_3)_6]Cl_4$ furnishes $.......$ ions.

Potassium ferrocyanide is a