Assertion (A) is true: The ionisation enthalpies of the early actinoids are lower than those for the early lanthanoids . Reason (R) is also true and correctly explains (A): When $5f$ orbitals are beginning to be occupied, they penetrate less into the inner core of electrons. Therefore, the $5f$ electrons experience greater (more effective) shielding from the nuclear charge by the inner core compared to the $4f$ electrons of the corresponding lanthanoids. Because these outer electrons are more effectively shielded, they are less firmly held by the nucleus, resulting in lower ionisation enthalpies . (Note: It is important to distinguish that while $5f$ electrons themselves provide poor shielding to other electrons, they experience greater shielding from the inner core).