Final answer:
Yeast's increased popularity signifies an increase in demand, not just the quantity demanded. This is represented by a rightward shift in the demand curve for yeast, which is option C from the provided choices.
Step-by-step explanation:
When yeast's popularity increases, it leads to a higher demand for yeast in the market. This increased popularity can be due to an increase in consumer tastes, preferences, or an increase in the overall need for yeast. According to economic principles, an increase in demand for a product means that consumers are willing to purchase more of the product at the same price, or the same amount at a higher price. This phenomenon is represented on a graph by a rightward shift in the demand curve for the product, meaning that the curve moves to the right, showing an increase in quantity demanded across all price points. Therefore, given the options provided, the correct answer is C. the demand for yeast and the demand curve for yeast shifted rightward.