== Manchester United F.C. == 

 The correct word is "that" in this case. Trust me, I'm a professional writer. Look at the sentence: "Manchester United Football Club is a professional football club based in Old Trafford, Greater Manchester, England, that plays in the Premier League." Then take out the bolded portion: "Manchester United Football Club is a professional football club that plays in the Premier League". If you replace 'that' with 'which', the sentence is no longer grammatically correct. –