The correct answer is: Aurangabad.
The 100 MW Aurangabad Solar Power Plant is the largest solar power plant in Bihar. It is located in the Aurangabad district of Bihar, India. The plant was commissioned in 2017 and is owned and operated by the Bihar State Power Holding Company Limited (BSPHCL). The plant uses crystalline silicon photovoltaic modules and has a capacity factor of 20%.
The Rohtas district is located in the south-central part of Bihar. It is the largest district in Bihar in terms of area. The district is home to the famous Rohtas Fort, which was built by Sher Shah Suri in the 16th century.
The Gaya district is located in the south-central part of Bihar. It is the second largest district in Bihar in terms of population. The district is home to the famous Mahabodhi Temple, which is a UNESCO World Heritage Site.
The Bhagalpur district is located in the south-eastern part of Bihar. It is the third largest district in Bihar in terms of population. The district is home to the famous Bhagalpur University, which is one of the oldest universities in India.
The above options are all districts in Bihar. However, the largest solar power plant in Bihar is located in the Aurangabad district.