Respuesta :
The subordinate clause here is 'to which another pronoun refers', and it is called a 'relative clause'.
In the sentence "an antecedent is a noun or pronoun to which another pronoun refers" the subordinate clause is "to which another pronoun refers".
It is a subordinate clause introduced by the subordinating conjunction "to which" that is linking the independent clause to the dependent one.
The subordinate clause is a relative (also called adjectival) clause since it is modifying the noun "pronoun".