TV Line announced on January 30, 2011 that Laura Vandervoort will return as Kara for one last time in one of the show’s final five episodes: Smallville Scoop: Supergirl Returns.
These are the air dates and episode titles of the final 5 episodes according to IMDB:
Season 10, Episode 17: Kent
Original Air Date—15 April 2011
Season 10, Episode 18: Booster
Original Air Date—22 April 2011
Season 10, Episode 19: Episode #10.19
Original Air Date—29 April 2011
Season 10, Episode 20: Prophecy
Original Air Date—6 May 2011
Season 10, Episode 21: Finale
Original Air Date—13 May 2011
I’m going to guess it’s the untitled episode, but it could any besides the Booster Gold episode.
Update: IMDB lists Prophecy as the second and final Season 10 episode for Laura.