Which part of the proposition denotes the relation between subject and predicate?

The part that shows the relation between subject and predicate is the copula.

A standard-form categorical proposition has four parts, quantifier, subject term, copula, and predicate term.

The copula is the verb of being, is, are, is not, or are not.

It joins the subject to the predicate and states whether the predicate is affirmed or denied of it.

That linking role makes the copula the bearer of the subject-predicate relation.