The answer is yes, the equation represents a proportional relationship.
The reason is that a proportional relationship is best described as that in which the value of one variable depends on what happens to the other variable. Just like having one more child every year would mean spending more money on education, etc.
In a proportiona; relationship as shown in this question, y is the total cost of a pizza and each x (each topping) would determine how much is y. So requesting for 5 more toppings would result in 1.5 multiplied by 5, and requesting 10 more would result in 1.5 multiplied by 10. So as the amount of toppings (x variable) increases, the total cost (y variable) likewise would increase.
As x increases or decreases, the value of y would likewise increase or decrease. That makes this equation a proportional relationship