N3−,O2− and F−are isoelectronics ( 10 electrons) species. Their ionic radii will decrease with increase in magnitude of the nuclear charge from N(Z=7) to 0(Z=8) and F(Z=9). i.e. N3−>O2−>F− 1.71Å‌1.40Å 1.71Å‌1.36Å Hence, option (a) is correct.