Final answer:
The correct approach is to target users who have viewed the product and exclude those who have purchased. This is done by positive targeting of product viewers and negative targeting of product purchasers.
Step-by-step explanation:
The targeting rule that displays an ad to users who have viewed a product but not purchased it is: Option 4) Positively target all product viewers AND negatively target all product purchasers.
By setting up a campaign this way, you are showing ads to those who have shown an interest in the product (those who viewed it), while excluding those who have already converted (purchased the product) from seeing the ad again.
This strategy relies on a combination of inclusion and exclusion to refine the audience to those who are most likely to make a purchase after having previously shown interest.