The correct answer is: A. Dhanbad
Dhanbad is the most populous district in Jharkhand, with a population of 3,450,652 as of 2011. It is also the most densely populated district in the state, with a population density of 2,500 people per square kilometer.
Dhanbad is located in the eastern part of Jharkhand, and is part of the Chota Nagpur Plateau. The district is known for its coal mines, and is home to the Dhanbad Coalfield, one of the largest coalfields in India.
Dhanbad is also a major industrial center, and is home to a number of steel mills, power plants, and chemical plants. The district is well-connected by road and rail, and is served by the Dhanbad Airport.
The other options are incorrect because they are not the most populous districts in Jharkhand. Bokaro has a population of 2,473,109, Sahibganj has a population of 1,427,846, and Khunti has a population of 1,229,471.