Name the member of the Actinoid Series which is the last element:

The correct answer is Option (3) → Lawrencium (Lr)

The actinoids are the fourteen elements that follow actinium (Z = 89), in which the 5f orbitals are progressively filled. They run from thorium (Z = 90) to lawrencium (Z = 103):

    Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr

Lawrencium has the configuration [Rn] 5f¹⁴6d¹7s²; with the 5f subshell completely filled, it is the last member of the series.

NCERT (2026-27 edition):
The Inner Transition Elements (f-Block)