The answer is ‘who’.
‘Who’ should be used–not ‘that’–when referring to people.
However, ‘who’ can also be used to refer to nouns such as countries, like this:
The nation whose people respect laws more than ethnicity can be a republic.
And the answer to the question ‘who said “A democracy if you can keep it?”‘ is Benjamin Franklin. The question he was asked, by a crowd which saw him departing from the Constitutional Convention, was ‘what kind of government have you given us?’