If a guest requests a mule that is not on the menu, what is the appropriate way to ring it in?

When a guest requests a mule that is not on the menu, the appropriate way to ring it in is to select the specific liquor they desire, add ginger beer, and garnish with lime. This aligns with the traditional preparation of a mule, which typically includes a base spirit (often vodka), ginger beer, and a lime wedge for flavor. By using this method, you ensure the drink is made correctly according to common cocktail standards, catering to the guest's request accurately.

The other options do not align with a mule’s preparation. A standard margarita recipe would not incorporate the correct ingredients for a mule, as it is a completely different cocktail. Choosing any vodka and adding tonic water alters the foundational elements of the drink, as tonic water does not contribute the same flavor profile as ginger beer. Asking the guest for a different drink option might not meet their specific desire for a mule and could lead to dissatisfaction, as it does not respect their initial request. Therefore, option A is the only appropriate response that honors the guest's request for a mule.