Explanation:

The primary reason for a drastic increase in population, especially in a region like Uttar Pradesh, is generally the improvement in health and control of diseases. This leads to a significant decrease in mortality rates, particularly infant and child mortality, while birth rates may remain high for a period, resulting in population growth. Increasing literacy rate, agricultural improvements, and urbanization can influence population dynamics but are not the direct primary drivers of a 'drastic increase' in overall population size.