Which state has the highest population density in North-East India according…

Population density is the number of persons living per square kilometre of area, calculated as total population divided by total geographical area. It is one of the standard demographic indicators published for every Census, including India's 2011 Census, and it varies sharply within India's North-East because the region mixes broad river valleys with dense hill and forest terrain.

Applying the 2011 Census figures to the four states named in this question gives the following population densities:

As a check, Assam's figure of about 398 persons per square kilometre is close to the all-India average density of about 382 persons per square kilometre recorded in the same Census, consistent with Assam being the most populous North-Eastern state and largely settled in the plains of the Brahmaputra valley, unlike the hillier terrain of Manipur, Mizoram, and Tripura.

Among the four states listed, Assam therefore has the highest population density in North-East India according to the 2011 Census.