Answer:
The idea that guarantee that it's referent exist outside the self is : an innate idea.
Step-by-step explanation:
Descartes described three ideas: adventitious, innate, invented. The one of them that is a garantee that referent exist outside the self is innate.
This idea according to Descartes, id born in a human, it is not learned for experience. God for instance would correspond to a innate idea. He explains that the body is a continuant of God. If there is an idea of God, there referent of it is outside.