Is The cost of purchasing x pounds of bananas for $0.55 per pound proportional

Yes, it's proportional.

Step-by-step explanation:

A function is proportional if the value increases or decreases at a constant rate. In this case, the price per pound will always be $0.55, so no matter how many pounds you buy the cost of the bananas will increase at a constant rate of $0.55 per pound.

Hope this helps :)