Looking at the graph and its missing parts, we can identify the following items to fill each box:
A. Population Size.
Since the population size is a dependent variable in relation to time, we should use it in the vertical axis.
B. Slow growth when population is small.
In the first part of the graph, the population is small and also grows slowly.
C. Faster and faster growth as population becomes larger.
In the final part of the graph, the population is big and also grows fast.
D. Time
Since the time usually is an independent variable, we should use it on the horizontal axis.