(1) The decrease in ionic radii of the lanthanides from 103 pm (La$^{3+}$) to 86 pm (Lu$^{3+}$) in the lanthanide series.
(2) The poor shielding of nuclear charge (nuclear attractive force on electrons) by 4f electrons
(3) The poor shielding of nuclear charge (nuclear attractive force on electrons) by 5d electrons
(4) The same effective nuclear charge from Ce to Lu
$\begin {array} {1 1} Both (2) and (3) \\ Only (2) \\ Both (1) and (2) \\ (1), (2) and (4) \end {array}$
