Which would be a likely cause of an increase in the demand for pizza? a reduced desire for take-out and fast-food dining a decrease in the price of hamburger sandwiches?

I think the correct answer to this would be:

"A health report showing eating pizza reduces stress"

If health news about pizza reducing stress would come out, people’s willingness to buy pizza would definitely increase. This is because of the additional positive reinforcer, reducing stress, associated with the great taste of pizza that people would definitely buy this.