The S-curve refers to logistic growth, and it occurs when the resources are limited because at some point the population will reach its carrying capacity (meaning that there woen't be resources available for it to grow even more).
The J-curve refers to exponential growth, in which there are resources enough available for the population to grow undefinitely, without reaching the carrying capacity.
As we can see, "S" and "J" are letters that refers to the shape of the curve.
Therefore, the correct answers are:
EXPONENTIAL GROWTH: A. when resources are unlimited
LOGISTIC GROWTH: B. when resources are limited
S-SHAPED CURVE: D. logistic growth
J-SHAPED CURVE: C. exponential growth