In Act III, what remedy corrects the crossed-loved couples?

The answer to this is that the remedy is the exact same
thing that caused all of the trouble in the first place.  As you know, all of the
trouble (all of the people falling in love with the "wrong" partners) was caused by
Puck.  He was the one who put the potion on the eyes of various people, causing them to
fall for the wrong person.

In Act III, Scene 2, Oberon
tells Puck to fix all the trouble.  So Puck gets Lysander and Demetrius to fall asleep
and then annoints their eyes.  When they wake, they see the "right" women and everything
is back to how it should be.