Which of the following elements form interstitial compounds?

Stainless steel does not rust because

What metals are in German silver alloy?

Given below are two statements:
Statement $I$: Ferromagnetism is considered as an extreme form of paramagnetism.
Statement $II$: The number of unpaired electrons in a $Cr^{2+}$ ion $(Z=24)$ is the same as that of a $Nd^{3+}$ ion $(Z=60)$.
In the light of the above statements,choose the correct answer from the options given below:

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

Why is the first ionisation enthalpy of $Cr$ lower than that of $Zn$?