Which of the following types of trust models is used by a PKI?

Transitive trust is a characteristic of directory services and Kerberos authentication in environments like Active Directory, not the defining trust model of PKI.

Open source is a software licensing and development model, not a security trust model used by PKI.

A decentralized trust model - where peers mutually validate each other without a central authority - describes PGP's web of trust, not PKI.

PKI relies on a centralized hierarchy where a root CA acts as the ultimate trust anchor, optionally delegating to subordinate CAs, all of which derive their trustworthiness from the root. Clients trust certificates by validating the chain up to a root CA that is already trusted. This centralized model contrasts with decentralized models like PGP's web of trust, where no single root authority exists.