To determine which Indian state had the lowest population density according to the 2011 Census, we need to understand what population density means. Population density is defined as the number of people living per unit area, typically per square kilometer.

The question presents us with four options: Meghalaya, Mizoram, Arunachal Pradesh, and Manipur. The correct answer is Arunachal Pradesh.

Let's briefly analyze each option:

• Meghalaya: Known for its hilly terrain, Meghalaya has a moderate population density that is higher than Arunachal Pradesh.
• Mizoram: Despite being hilly, Mizoram's population density is higher than Arunachal Pradesh, owing to smaller overall land area compared to its population.
• Arunachal Pradesh: As the largest state in the North-Eastern region, it has a vast geographical area but a small population, resulting in the lowest population density in India.
• Manipur: This state is more densely populated than Arunachal Pradesh due to its relatively smaller area and larger population.

The key point is that Arunachal Pradesh has a vast area with a significantly low number of inhabitants per square kilometer compared to other states, especially among the ones listed in the options.

Thus, the correct answer is Arunachal Pradesh, which had the lowest density of population according to the 2011 Census data.