The correct answer is (a).
Initially, the adulterated rice has 5 kg * 2% = 0.1 kg of stones. After half of the stones are removed, the adulterated rice has 0.1 kg / 2 = 0.05 kg of stones. The percentage of stones in the adulterated rice is 0.05 kg / 5 kg * 100% = 1%.
Option (b) is incorrect because it is the percentage of stones in the original adulterated rice. Option (c) is incorrect because it is the percentage of stones in the adulterated rice after all of the stones are removed. Option (d) is incorrect because it is the percentage of stones in the adulterated rice after half of the stones are removed.