The correct sequence of the median age of populations for India, China, the UK, and the USA is captured in option 2: India < China < USA < UK.

To clarify, the median age represents the age that divides a population into two numerically equal groups - that is, half the people are younger than this age and half are older.

Starting with India, it`s well-known as a young country with a lower median age than the other countries. China, having a larger and older population due to its one-child policy, has a higher median age than India but below USA and UK due to recent advancements and a relatively lower growth rate.

The USA, a developed nation with balanced birth and death rates, has a higher median age than China and India but is less than UK.

Lastly, the UK has the highest median age among these. The higher life expectancy, better healthcare, and low fertility rate contribute to an older median age.

This explanation should provide a clearer understanding of the pattern identified in the correct answer.