Which of the following weighs less when weighed in a magnetic field?

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ The atomic radii of the elements of the second transition series $(4d)$ are larger than those of the first transition series $(3d)$.
$(b)$ $Cr^{+}$,$Mn^{2+}$,and $Fe^{3+}$ have the same electronic configuration.
$(c)$ $Cr^{+}$,$Mn^{2+}$,and $Fe^{3+}$ have the same $d^5$ electronic configuration.

Variable valency,in general,is exhibited by

An ion which has $18$ electrons in the outermost shell is

The metal ion (in gaseous state) with the lowest spin-only magnetic moment value is .... .

Amongst the following,the lowest degree of paramagnetism per mole of the compound at $298 \ K$ will be shown by: