Which one of the following properties is not a property of transition elements?

Which of the following ions has the maximum magnetic moment?

The magnetic moment of $Mn^{4+}$ ion is approximately ...... $B.M.$

Out of $Cu_2Cl_2$ and $CuCl_2$,which is more stable and why?

Fill in the blanks:
$(a)$ The $4f$ and $5f$ series elements are known as lanthanoids and actinoids,respectively.
$(b)$ In transition elements,electrons are filled in the $d$-orbitals of the .......... energy levels.

The second ionisation energy of the following elements follows the order