Answer: Orange.
Explanation:
The graph of the slopes of the red function is equal to the derivate of the red function.
First, let's analyze the red function (from left to right).
First we can see that the red function is positive, and it decreases (so we must have a negative slope here).
Then we can see a little inflection point where the red function seems to be constant for a bit (here the slope must become zero for a little bit).
Then the red function keeps decreasing (so the slope is negative again).
Then the graph of the slope is:
Negative, then it increases to zero, and it decreases again.
The only graph that does this is the orange one, so the answer is orange.