Hi!
The correct answer is B. The value of M is a product of N and a factor that does not depend on N.
That is true because the relation is : M(t) = N * (1+r)^t , N is N and (1+r)^t is a factor that does not depend on N.
Also, because none of the other answers is true, it makes easier for B to be the one true answer, explanation:
A. The initial value is (1+r) - WRONG, the initial value is N, it says that
C. The function is linear - WRONG, we have exponential growth here, because of the factor (1+r)^t
D. N increases each month - WRONG, N is the initial number of members and it does not change.
Hope this helps!