What is the oxidation state of cobalt in $[\text{Co}(\text{NH}_3)_6]^{3+}$ ?

Identify reagent B in following reaction.
$\text{R}-\text{X} \xrightarrow{\ \large{\text{B}}\ } \text{Nitroalkane}$

Which of the following $n$ mole compound is obtained when $2n$ moles of $\text{C}_6\text{H}_5\text{Cl}$ treated of with $2n$ mole sodium atom in dry ether?

Which from the following compounds is not obtained when methyl bromide is treated with ethyl bromide in presence of sodium metal in dry ether?