Among the following systems, which will have the minimum radius for...

The radius of the $n$-th Bohr orbit for a hydrogen-like species is given by:

$r_n = \dfrac{0.529 \times n^2}{Z}$ \AA

For the first orbit, $n = 1$. Thus, the radius is inversely proportional to the atomic number $Z$ ($r_1 \propto \dfrac{1}{Z}$).

To have the minimum radius, the system must have the maximum atomic number $Z$.

For doubly ionized lithium ($Li^{2+}$), $Z = 3$
For deuterium atom ($D$), $Z = 1$
For singly ionized helium ($He^+$), $Z = 2$
For hydrogen atom ($H$), $Z = 1$

Since $Li^{2+}$ has the highest atomic number ($Z = 3$), it will have the minimum radius for the first orbit.