Which statement is true about the transition elements?

The electronic configuration of gadolinium (atomic no. $64$) is

Which of the following pairs has similar ionic radii?

Assertion $(A)$: While going from left to right in the periodic table,the atomic size decreases more rapidly for the $3d$-series compared to the $4f$-series of elements.
Reason $(R)$: $3d$-electrons experience lesser shielding than $4f$-electrons.

The most abundant transition metal in the earth's crust is

The magnetic nature of elements depends on the presence of unpaired electrons. Identify the configuration of transition elements which shows the highest magnetic moment.