The correct answer is A. 1st.
Bihar is the most populous state in India, with a population of 104,099,192 as of 2011. It has a population density of 1,106 persons per square kilometer (2,860/sq mi), which is the highest among all Indian states.
The other options are incorrect.
Option B is incorrect because the third most populous state in India is Uttar Pradesh, with a population of 199,581,477 as of 2011.
Option C is incorrect because the fifth most populous state in India is Maharashtra, with a population of 112,374,333 as of 2011.
Option D is incorrect because the sixth most populous state in India is West Bengal, with a population of 91,347,736 as of 2011.