From " n' " (="ne") and "que". Here, "ne ... que" is used to mark the restriction. This is equivalent to "seulement" (="only"). Example: "il ne mange que des fruits" (~ "il mange seulement des fruits") -> "he eats only fruit". "ne" is placed before the verb, "que" before the object of the restriction.

To add to DavidGi's good reply, "ne ... que" is often translated "nothing but". "He has done nothing but that."

The sentence is in the past, so the English pronoun usually is 'that' as something removed from here and now. That said, translating 'cela' as 'this' would work if the verbs were translated as 'has lived/has done'. So maybe Duolingo should be alerted to the possibility.