Which of the following transition metals is considered the softest?

Assertion : Cuprous ion $(Cu^+)$ has unpaired electrons while cupric ion $(Cu^{2+})$ does not.
Reason : Cuprous ion $(Cu^+)$ is colourless whereas cupric ion $(Cu^{2+})$ is blue in the aqueous solution.

Transition elements form complex compounds due to:

What is the value of the spin-only magnetic moment of $Ni$ $(Z=28)$ in the $+2$ oxidation state (in $BM$)?

Which of the following transition metal ions has a definite magnetic moment?

On moving from left to right in a period within the $d$-block transition metals,how do their atomic volumes change?