According to Russell, are sense data NOT the only things immediately known, or are they not purely mental?

Which theory holds the notion that proposition are true when they correspond to reality or the world?

the Correspondence Theory

Historically, the most popular theory of truth was the Correspondence Theory. First proposed in a vague form by Plato and by Aristotle in his Metaphysics, this realist theory says truth is what propositions have by corresponding to a way the world is.