By what means does Edmund prompt Edgar to flee in this scene?
By telling Edgar that his astrological charts portend disaster and urging him to leave the country while he still can.
By telling Edgar that both the Duke of Cornwall and the Duke of Albany are on their way to challenge him to combat.
By claiming that the fifty knights of Lear’s whom Goneril has just dismissed are on their way to Gloucester’s castle, outraged and drawing their swords on whomever they encounter.
By contending that it would be more cowardly to stay, since the trouble is overseas, where Cordelia is – and where the dukes are now headed.
By suggesting to Edgar that he may have (wittingly or not) antagonized one of the dukes, and then staging a sword fight while appearing to have Edgar’s best interests at heart.