Which of the following will have the smallest radius of first orbit (n = 1)?

Doubly ionized lithium

The correct answer is Option (4) → Doubly ionized lithium

Radius of the first orbit (n = 1) of an atom be determined using Bohr's Radius formula -

$r=\frac{n^2ε_0h^2}{\pi me^2Z}⇒r∝\frac{1}{m}$

Mass of double ionized lithium = 6 ($He^{2+},H,2H$)

∴ Smallest radius is of $He^{2+}$