No clear RAW answer, but several options for how to adjudicate the attack roll.
Nothing in the spell's description or general rules clearly defines what should happen in this situation. As such there are a few options that I will present, and I think you just need to see what works best for you and your players.
Things that are definitely RAW first
This "monk" cannot take the Dodge action. It can certainly look like it is dodging, but nothing in the spell description allows it to actually take actions. As such any RAI-based attack roll should not have disadvantage due to this "Dodging".
Physical interaction with the image reveals it to be an illusion, because things can pass through it
If the orc's sword-stroke hits the illusion, its illusory nature will be revealed.
Major Image is a 3rd level spell, and the highest level spell that allows you to create any generic creature as your illusion (Phantasmal Killer and Illusory Dragon are higher level, but less generic). As such, the image created is definitely intended to appear life-like - including doing things like appearing to try to evade blows in battle.
Although the image may look like a monk, it's just an illusion controlled by a Wizard. There's no reason to involve the stats of a monk here.
- Any attack hits the illusion. There is nothing in the spell description that suggests the ability to avoid attacks, so the image cannot do so.
- AC = 4. This is the AC of a creature with Dexterity = 0 and no hardness to otherwise increase its AC. Basically the same as option 1 but including the ability of other creatures to miss. Practically speaking almost every creature has an attack bonus of at least +4, so this will only cause them to miss on critical fumbles.
- AC = 10. This is the AC of a creature with 0 or n/a dexterity.
- AC = 10 + something representing the "Dexterity" of the illusion. Perhaps the caster's spellcasting modifier.
I have only presented options to determine the AC of the illusion or let it get automatically hit. In my view if a creature makes an attack it should be adjudicated as an an attack roll. Not least because if the tables were turned it not being an attack roll would give your players metagame knowledge that they were fighting an illusion!