"Strong fear inducing ad" is one of the levels of the Independent Variable. This ad is what the experimenter showed the participants assigned to the experimental group. In this case it seems the strong ad would be the experimental group. But if you saw it as the control group that is not a problem with your analysis of this study since you did not have enough information to decide which is the experimental and which is the control group. The general label for the independent variable would be Ad Fear with two levels.
"low fear inducing stun gun ad" is the other level of the Independent Variable (IV); it is the control group since the experimenters wanted to see if the strong fear ad would lead to more people saying they would buy a stun gun. They compared the strong fear group's rating to the low fear ad group's rating of how likely they were to buy the stun gun.