Option A is incorrect. Statement 4 mentions that rural-urban migration is taking place regularly in the country. However, migration within the country does not contribute to an increase in population as it is not adding new individuals to the country. Therefore, rural-urban migration cannot be the cause of increased population growth.

Option B is incorrect. The decline in the death rate alone cannot explain the increase in population growth. Both the birth rate and death rate play important roles in determining population growth, and the decline in the death rate needs to be considered in conjunction with the birth rate.

Option C is incorrect. The birth rate alone cannot be the cause of the increase in population growth. The death rate also plays a significant role, and the increase in population is due to the decline in the death rate rather than a change in the birth rate alone.

Option D is correct. The rate of population growth is increasing because the death rate is declining faster than the birth rate, which directly supports the conclusion in Statement 2.