Which of the following is a strong field ligand?

The $IR$ stretching frequencies of free $CO$ and $CO$ in $[V(CO)_6]^-$,$[Cr(CO)_6]$ and $[Mn(CO)_6]^+$ are $2143 \, cm^{-1}$,$1860 \, cm^{-1}$,$2000 \, cm^{-1}$ and $2090 \, cm^{-1}$,respectively. Then the correct statement about metal carbonyls is:

$A$ pink coloured aqueous solution of $Co(NO_3)_2$ turns blue on the addition of $HCl$ gradually. This colour change happens due to the formation of

Complexes of $CN^-$ ligand are generally...

The colour of cobalt chloride solution is:

What is the coordination entity formed when excess of aqueous $KCN$ is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when $H_{2}S_{(g)}$ is passed through this solution?