There is only one fertiliser plant in Assam which is located at

Tuli in Nagaland
Gangtok in Sikkim
Nangpo in Meghalaya
Namrup in Assam

The correct answer is D. Namrup in Assam.

The Namrup Fertiliser Plant is a public sector undertaking under the Ministry of Chemicals and Fertilizers, Government of India. It is located in Namrup, Assam, India. The plant was commissioned in 1976 and has a capacity of 1,200,000 tonnes per annum of urea. The plant is a major source of urea for the Indian market.

The other options are incorrect. Tuli is a town in Nagaland. Gangtok is the capital of Sikkim. Nangpo is a village in Meghalaya.