The answer would be:
B. The speed of the electric fan.
This is because:
An independent variable is what influences changes in the dependent variable. So you can say that it is the variable that we chance. It is called INDEPENDENT because it is not affected by the dependent variable.
In this case, the speed of the electric fan is not affected by the graham cracker, but the speed of the electric fan AFFECTS the graham cracker.
(Although the biggest clue is the variable that is manipulated or changed by the experimenter.)