Determine whether the relationship between time and the amount of popcorn popped is directly proportional?

Directly proportional implies that the rate of popcorn popping over time is constant; if t minutes go by, exactly p ounces of popcorn will pop, and this pattern will continue forever (or at least until all of the popcorn is gone). If you've ever made microwave popcorn before, you know that this is totally untrue. Most of the popcorn pops early on, leading to the steep curve you see on the left side of the graph. As more and more time goes by, more popcorn pops, which naturally means that there are less popcorn kernels left to pop as time goes on - of course, this means that you'll have fewer ounces of popcorn popping in some interval of time further to the right of the graph than you would during that same interval further to the left of the graph.

Your answer here would be that fourth option, the graph does not make a straight line, since a straight line would imply a directly proportional/linear relationship.