The correct answer is: c) A rapid increase in urbanization.
Mizoram has seen a rapid increase in urbanization since the 1951 Census. The percentage of the population living in urban areas has increased from 13.5% in 1951 to 37.4% in 2011. This is due to a number of factors, including economic development, migration, and the expansion of infrastructure.
The rapid increase in urbanization has had a number of impacts on Mizoram. It has led to an increase in the demand for housing, water, and other services. It has also led to an increase in pollution and traffic congestion. However, it has also created new opportunities for employment and education.
The other options are incorrect. Option a) is incorrect because the population density of Mizoram has increased since the 1951 Census. Option b) is incorrect because the sex ratio of Mizoram has remained relatively stable since the 1951 Census. Option d) is incorrect because the literacy rates of Mizoram have increased since the 1951 Census.