Final answer:
A mystery shopper could carry out all the research tasks listed, including recording bathroom cleanliness, price negotiation, preventing age-inappropriate game sales, and assessing product quality like jelly distribution in doughnuts.
Step-by-step explanation:
A mystery shopper could indeed perform all of the research tasks mentioned: recording the cleanliness of bathrooms, negotiating with sellers on the price for an item, ensuring that children cannot buy M-rated video games, and recording the evenness of jelly in a doughnut.
All these tasks are common activities for a mystery shopper, whose role is to provide objective feedback on various aspects of the consumer experience, often without the knowledge of the staff or management of the business being evaluated.