Final answer:
The cashier should suggest option A, a dessert, as it complements the combo meal that already includes a chicken sandwich, fries, and a drink. Desserts usually follow the main course, making it an appropriate and enticing addition to offer to the customer.
Step-by-step explanation:
A customer who has ordered a combo meal with a chicken sandwich, medium fries, and a medium drink might be interested in adding a dessert to complete their meal. Suggesting a dessert as an additional item is an excellent option because desserts are typically consumed at the end of a meal. Therefore, option A, suggesting a dessert, seems to be the most appropriate choice for the cashier to make.
While suggesting an additional sandwich or appetizer could be seen as enhancing the meal, these items are generally seen as more substantial and could be too filling given that the customer has already ordered a complete meal. An enlarge-it option might be appealing for those looking for a larger portion of their existing meal components, but it's not a new item, just more of what they already have.