The correct answer is C. Idukki.
Mullaperiyar Dam is a masonry dam located in the Idukki district of Kerala, India. It is the largest arch dam in India and the second largest in Asia. The dam was constructed across the Periyar River to provide irrigation and drinking water to the state of Kerala. It also generates hydroelectric power.
The dam is located in the Idukki district of Kerala, India. The district is located in the Western Ghats mountain range and is known for its scenic beauty. The district is also home to the Idukki Dam, which is the largest arch dam in India.
The other options are incorrect because they are not districts in Kerala. Thrissur is a district in Kerala, but the Mullaperiyar Dam is not located in Thrissur district. Pathanamthitta is a district in Kerala, but the Mullaperiyar Dam is not located in Pathanamthitta district. Alappuzha is a district in Kerala, but the Mullaperiyar Dam is not located in Alappuzha district.