. . . . . . . . problem of fundamental importance for . . . . . . . . developing countries is that of slowing down . . . . . . . . population growth.

Answer: Option B

The first blank shall take 'a' as the sentence talks of only one 'problem'.
The second blank shall take 'the' as it is followed by an adjective + a plural noun.
The third blank won't take any article as 'population growth' is adjective + an abstract noun.