What is being informally tested here, and what is the hypothesis test?

The Kenton Food Company is interested in testing which size of a new organic, whole-grain breakfast cereal is most appealing to consumers. The company experimented with releasing the product in different package sizes for a preliminary period before the product's mass release. The Chief Data Scientist of Kenton developed a regression model relating package size (in ounces) to product sales volume (in dollars). The diagnostic plot below was obtained. use this information to answer the following questions.