She was advised by her coach to ____ to form if she wanted to do well in the tournament.

The phrasal verb 'get back to form' means to return to one's previous good level of performance, especially in sports. A coach advising a player to do well in a tournament would naturally tell them to 'get back to form'. The other options — 'get on', 'get along', and 'get done' — do not collocate naturally with 'form' in this context.