Which of the following ions are diamagnetic?

The increase in the atomic radii of the third $(5d)$ series of transition elements is very small, which may be accounted for by the filling of '$X$' orbitals before '$Y$' orbitals. $X$ and $Y$ are:

In the first transition series,the highest $B.P.$ and $M.P.$ is of

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

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ Transition elements exhibit variable oxidation states.
$(b)$ The $E^o$ values for $Mn, Zn,$ and $Ni$ are more positive than expected from the general trend.
$(c)$ Any metal exhibits its maximum high oxidation state in oxide and chloride compounds.

Which of the following statements is correct?