The conclusion of the argument is: governments should ban all advertisements that promote smoking. Why? Cigarette smoking has been shown to be a health hazard.
We are asked to find a principle that strengthens the argument.
Let's take a look at answer choice (E):
"Advertisements should promote only healthful products."
We know only introduces a necessary condition, so this statement would be diagrammed as follows:
A = advertisement PHP = promote healthful products
A ==> PHP not PHP ==> not A
Since we know that cigarettes are a health hazard, we know that cigarettes are not a healthful product. Though it is true that not being a health hazard doesn't necessarily mean that something is healthful, we can infer that something that is a health hazard is definitely not healthful.
If we follow the contrapositive of answer choice (E), we know that if an advertisement is not promoting a healthful product, then there is no advertisement, which clearly strengthens the argument's conclusion by tying the premise (i.e. a health hazard) more closely to the conclusion (i.e. "government should ban all advertisements that promote smoking").
Hope this helps! Please let us know if you have any other questions.
