Explanation:

A population census is a comprehensive enumeration of the population of a country at a specific time, collecting demographic, social, and economic data. The regularity of a census is crucial for accurate planning and policy-making. \nOption A, '2 years', is incorrect. A census conducted every two years would be a very frequent and resource-intensive exercise, not typically followed by large nations like India. \nOption B, '5 years', is incorrect. While some countries conduct censuses or large-scale surveys every five years, this is not the practice for the full population census in India. \nOption C, '7 years', is incorrect. There is no standard census cycle of seven years in India or most major countries. \nOption D, '10 years', is the correct answer. In India, the population census is conducted decennially, meaning once every $10$ years. The last census was conducted in $2011$, and the next one was scheduled for $2021$ but has been delayed. This decennial cycle is a long-standing tradition and is common in many countries worldwide, providing a consistent interval for tracking demographic changes and trends.