If a creature makes an attack roll or an ability check with advantage, how is it affected by the bard's Cutting Words?
a) The number rolled by the bard is substracted from the highest of both d20, leaving two results from which the creature can still take the highest?
b) The creature takes the highest of both d20 then substracts the number rolled by the bard?
The relevant rules are:
Advantage and Disadvantage (PHB p.171)
When that happens, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage.
Cutting Words (PHB p.54)
When a creature that you can see within 60 feet of you makes an attack roll, an ability check, or a damage roll, you can use your reaction to expend one of your uses of Bardic Inspiration, rolling a Bardic Inspiration die and subtracting the number rolled from the creature’s roll.
Specific beats general in D&D 5e so the Advantage/disadvantage rule, being the more general, is applied first. Therefore, "the creature’s roll" in the Cutting words description is the end result of the advantage/disadvantage mechanic - there is only one "creature's roll" even though two dice are used in generating it.
This is the mechanic for all applications of bonuses/penalties to a roll irrespective of if they are static (like proficiency modifier) or random (like Bless)
Advantage allows you to use the higher of two rolls, then all modifiers are applied.
Use the higher of the two rolls if you have advantage
Note that the rule talks about the roll, not the final result.
As the lower roll is discarded, modifiers to it are moot. I think this corresponds to your version b). You can't take the lower rolled number without modifiers, as in your scenario a).
