You have been granted the magic fingers of extinction, one time use only. Snap them and name one species out loud and it will disappear forever from the face of the earth. Will you use this power? Upon what?
And note: we’re talking about a species - you might get away with several closely-related/allied species if you say ‘camels’ or ‘cows’, but if you say "bugs’ or ‘bacteria’, it just isn’t going to work, and by ‘isn’t going to work’, I actually mean the whole universe will explode, so be careful.