The two practices considered unethical are:
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The manager for a mobile phone provider posts negative reviews of a competing service. - This is unethical as it involves using one's position to harm a competitor rather than provide honest feedback or reviews.
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The executive for a fast food chain posts a rave review of his company’s new sandwich. - This can also be considered unethical as it may present a conflicted interest, where the executive is not providing an unbiased opinion, and could mislead consumers about the quality of the product.
The other two options (the manager of a bookstore writing a negative review of a cleaning service and the employee of a big box store posting a positive review of a movie) do not involve a conflict of interest in the same way and can be considered more acceptable.