What happens when acetaldehyde reacts with Fehling solution or Benedict's solution? 

Carboxylic acid is formed 

Fehling solution is an alkaline solution of of cupric ions in the presence of Rochelle salt (sodium potassium tartarate), whereas Benedict solution is an alkaline solution of cupric ions in the presence of citrate. Both these solutions when reacts with acetaldehyde produces carboxylic acid.