Final answer:
Option A). Your guest just ordered a Bombay Sapphire, up (↑), garnished with an onion. In short order, what did your guest just order? The guest just ordered a Martini, specifically a Gibson, which is a variation of a Martini garnished with an onion.
Step-by-step explanation:
Your visitor has placed an order for a Bombay Sapphire, up (↑), with an onion garnish. What your guest just ordered is a Martini, to put it briefly. When a drink is ordered "up," it indicates that it will be served in a cocktail glass as opposed to a rocks glass. Bombay Sapphire is a brand of gin. A classic martini is typically made with gin and vermouth and is topped with a twist of lemon or an olive. But in this instance, the visitor asked for a Gibson—a variation on the Martini—to be garnished with an onion.