Step 1: Calculate the increase in number of towns/UAs for each decade.
1961-1971: 2,590 - 2,365 = 225 towns
1971-1981: 3,382 - 2,590 = 792 towns
1981-1991: 4,652 - 3,382 = 1,270 towns
1991-2001: 5,161 - 4,652 = 509 towns
Step 2: Identify the lowest increase.
Comparing the increases:
1961-71: 225 towns
1971-81: 792 towns
1981-91: 1,270 towns
1991-2001: 509 towns
The lowest increase is 225 towns during the decade 1961-1971.
Answer: The decade 1961-1971 recorded the lowest increase in the number of towns/urban agglomerations with an addition of only 225 towns.