The correct answer is C. Samastipur.
Kahalgaon Super Thermal Power Station is a coal-fired power station located in Kahalgaon, Samastipur district, Bihar, India. It is owned and operated by NTPC Limited. The power station has an installed capacity of 2,100 MW. It was commissioned in 1989.
Option A is incorrect because Patna is the capital of Bihar. It is not a district of Bihar.
Option B is incorrect because Bhagalpur is a district of Bihar. It is not the location of Kahalgaon Super Thermal Power Station.
Option D is incorrect because Siwan is a district of Bihar. It is not the location of Kahalgaon Super Thermal Power Station.