Final answer:
A lemon twist is the traditional garnish for a martini when someone requests a twist, although other citrus options like lime or orange can be used if specified.
Step-by-step explanation:
When someone orders a twist as a garnish to their martini, the kind of twist that is usually used is b) Lemon. A lemon twist is the most classic choice for a martini. However, it's not uncommon for bartenders to use other types of citrus if requested or to complement a specific cocktail's profile. That said, traditions and cocktail recipes typically call for a lemon twist when referring to a 'twist.'