We know that the radius (r) of an orbit of an atom of atomic number Z, according to Bohr's Model is:

Where, n= Principal Quantum number and Z= Atomic Number.

Thus, for a fixed n (n=1), the radius of an orbit is inversely proportional to Z.

Among the options, lithium ion has the highest atomic number, Z=3.

Hence, option (d) is the correct answer.