Which of the following has the maximum number of unpaired electrons?

Which of the following forms a colourless solution in aqueous medium?

In the electronic configuration of which of the following elements is an electron arranged in the $5d$ orbital?

The electronic configuration of copper $({_{29}}Cu)$ is

Many transition metals form interstitial compounds. The characteristics of these interstitial compounds are:
$I$. They have high melting points,higher than those of pure metals.
$II$. They are very hard.
$III$. They retain metallic conductivity.
$IV$. They are chemically more reactive than the pure metals.

Addition of non-metals like $B$ and $C$ to the interstitial sites of a transition metal results in the metal becoming: