Given below are the two statements: one is labeled as Assertion (A) and the other is labeled as Reason (R).
Assertion (A): 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.
Reason (R): covalent and ionic radii in a particular oxidation state increases down the group.
In the light of the above statement, choose the most appropriate answer from the options given below:

Assertion (A): "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 TRUE. The covalent radii are approximately: N (70 pm), P (110 pm), As (121 pm), Sb (141 pm), Bi (148 pm). There is a large jump of about 40 pm from N to P (adding a new shell from period 2 to period 3). However, from As to Bi, the increases are much smaller because the addition of d-electrons (and f-electrons for Bi) causes poor shielding, which leads to a higher effective nuclear charge. This partially offsets the increase in radius due to additional shells.

Reason (R): "Covalent and ionic radii in a particular oxidation state increases down the group."

This is TRUE as a general trend. As we go down a group, new electron shells are added, so the atomic/ionic radii generally increase.

Relationship between A and R:

While R states that radii increase down the group (which is correct), it does not explain the specific observation in A -- namely, why the increase is large from N to P but small from As to Bi. The explanation for that requires the concept of poor shielding by d and f electrons, which is not mentioned in R.

Therefore, both (A) and (R) are true, but (R) is NOT the correct explanation of (A).