1... d4

Or 1...e4.

Which of these obviously illegal moves would you play as black?

funnily enough, i think this is quite interesting.

I think i'd go for e5 topping the kings knight , while trying to remove it with f3 is a bit awkward and d3 may lead to an exchange of queens endgame after say nf6 which i like the look of.

its really a question of which advance is more useful in theory, just an odd way of having it posed lol

this is interesting, I think I'd tend to go with the e pawn, maybe because I love playing advance french 

I saw the title and I missed the illegal part. Good one Ivandh  rofl.