Hint: There is some exception in the atomic radius of group 13 elements.
Step 1: Atomic radii of group 13 elements
$$B$$: Boron has the smallest atomic radius among group 13 elements.
$$Ga$$: In $$Ga$$, the $$d$$ orbital does not shield the nucleus effectively. So the effective nuclear charge is higher for the outermost electrons. So, the atomic radius is smaller.
$$Al$$: Down the group, the effective nuclear charge decreases as the number of shells increases, So the atomic radius of $$Al$$ is larger.
$$In$$: In $$In$$, the $$d$$ orbital does not shield the nucleus effectively. So the effective nuclear charge is higher for the outermost electrons. But down the group, the effective nuclear charge decreases as the number of shells increases. So, these two effects result in the atomic radius of $$In$$ similar to the atomic radius of $$Al$$.
Step 2: Atomic radius order
$$B<Ga<Al\mathtt{\sim}In$$
Final Step: Correct option is (D) $$B<Ga<Al\mathtt{\sim}In$$