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

where n= Principal Quantum Number

For shortest orbit, n=1 which means it is fixed.

For hydrogen, Z=1. So, r=0.529 A°. So, option (a) is wrong.

For deuterium, Z=1. So, r=0.529 A°. So, option (b) is wrong.

For helium ion, Z=2. So, r=0.2645 A°. So, option (c ) is wrong.

For lithium ion, Z=3. So, r=0.176 A° ~ 18pm. So, option (d) is right.

Hence, option (d) is the correct answer.