There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and/or f orbitals in heavier members.
As going down in a group, the number of shells increases, hence the covalent and ionic radii increases.
Both (A) and (R) are true but (R) is not the correct explanation of (A)
But the correct explanation is the presence of completely filled d and f-orbitals of heavier members.