With which words does Caesar acknowledge his sad surprise that Brutus is part of the conspiracy? Veni, vidi, vici. This was the noblest Roman of them all. Beware the Ides of March. Et tu, Brute?
The answer would be "Et tu, Brute?". This famous line translates to "Even you, Brutus?", which displays Caesar's shock and surprise that Brutus was part of the conspiracy.