The correct answer is option 'A': 25 thousand.

Explanation:
The density of population is a measure of the number of individuals living in a specific area. In this case, we are looking at the density of population per square kilometer in Kolkata, as per the 2011 census.

The 2011 census is a reliable data source that provides information about the population of a particular area. Kolkata is a major city in the eastern part of India and is known for its cultural and historical significance.

According to the 2011 census, the population of Kolkata was around 4.5 million. To calculate the density of population, we divide the total population by the area of the city. However, the area of the city itself is not provided in the question.

To find the area, we can refer to the official website of the Kolkata Municipal Corporation, which states that the area of Kolkata is approximately 185 square kilometers. Now we can calculate the density of population per square kilometer.

Density of population = Total population / Area

Density of population = 4.5 million / 185 square kilometers

Simplifying the calculation, we find that the density of population in Kolkata is approximately 24,324 individuals per square kilometer. Rounding off this value gives us the answer, which is 25 thousand individuals per square kilometer (option A).

In conclusion, the density of population per square kilometer in Kolkata, as per the 2011 census, is approximately 25 thousand. This indicates a high concentration of individuals living in a relatively small area.