The smallest rate of increase in urban population in a decade during 1921-1961 occurred in

Let's solve the options one by one ,

Option A :Rate of increase of urban population during 1951-61 = $$\dfrac{79-62}{62}$$ = 0.274

Option B :Rate of increase of urban population during 1941-51 = $$\dfrac{62-44}{44}$$ = 0.409

Option C :Rate of increase of urban population during 1931-41 = $$\dfrac{44-33}{33}$$ = 0.33

Option D :Rate of increase of urban population during 1921-31 = $$\dfrac{33-28}{28}$$ = 0.1785

Among the above values smallest rate of increase was during 1921-31

Hence D is the correct answer.