a) Each missile hits a copy of the bad guy, destroying any that are images and damaging the real adept if they hit him. Result: Hurt adept, fewer images
b) Each missile fizzles out if it is sent at an image or hits if its sent at the adept. Result: Hurt adept, same number of images
or c) the magic missiles cannot even target the figments at all (since they are not valid targets of the spell), and the casting mage can ONLY target the real Mage concealed by the Mirror Image. Result: Mage REALLY hurt, no images lost.
d) magic missiles launch, but ignore images and only hits the mage, I think, is not a real option- it doesn't comport with the spell targeting rules (PHB p175).
Looking at those rules, I see nothing that says that a spell fizzles if you choose an invalid target.
In fact, the only WoTC product I know of that
explicitly says a spell fizzles (dissipates harmlessly) for lack of a proper target is Magic: The Gathering.
It is a concievable ruling that a spell that requires a specific kind of target does not fizzle, but rather cannot be cast, and the spellcaster does not lose that spell.
Ask yourself this: If a Mage cast Magic Missile at a single illusory Ogre, can it even target the illusion? It isn't a creature. If the spell fizzles or cannot be cast, the mage can immediately tell his allies that he has just targeted an illusion.
The only way to get around this "Magic Missile as illusion detector" because it must target creatures, not illusory creatures is to rule that MM can target what the caster
percieves to be a creature...including inanimate objects that he cannot see very well- obscured by shadow, or fog, etc.