Which of the following is a coloured compound?

The melting point of $Zn$ is lower as compared to those of the other elements of the $3d$ series because:

Variable valency,in general,is exhibited by

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.

Which one of the following characteristics of the transition metals is associated with their catalytic activity?

The electrode potential of $M^{2+} / M$ for $3d$-series elements shows a positive value for which of the following?