I think that there is a number missing here:

in the sense that

of the 24 members alreaday support the idea.

of 24 is
which means that 8 members already support the idea.
So if they need the support of 13 members, this means that they need 13-8=5, that is 5 more members.
Answer:
the concept of the sustainable development come into existence in 1946
C. Communities direct and manage their own schools
B, since all of the other answers would make the amount of the product go down.