The two options that are ethical for Carl to post are:
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A positive review of his own product in which he describes the product's uses - This is ethical as long as he is honest about the product and its uses.
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A positive review of a tool he used in creating his invention - This is ethical because he is honestly sharing his experience with a tool that he has actually used.
The other options involve unethical practices, such as posting a negative review of a rival product he has not used or posting a negative review of a potato chip flavor based on personal taste without a valid reason to discuss it in the context of his invention.