The correct answer is: A. 40 : 60
The rural-urban population ratio in Bihar is 40:60. This means that 40% of the population of Bihar lives in rural areas, while 60% lives in urban areas.
Option B is incorrect because it states that the ratio is 20:80. This would mean that only 20% of the population of Bihar lives in rural areas, which is not accurate.
Option C is incorrect because it states that the ratio is 50:50. This would mean that half of the population of Bihar lives in rural areas and half lives in urban areas, which is also not accurate.
Option D is incorrect because it states that the ratio is 80:20. This would mean that 80% of the population of Bihar lives in rural areas, which is not accurate.