Which of the following security measures uses certificates to verify identity?

The correct answer is Public Key Infrastructure (PKI) because it is designed specifically to manage digital certificates and public-key encryption, enabling secure communications and verifying identities. In a PKI setup, certificates serve as a trusted method to associate a public key with an individual or entity's identity. This verification process ensures that the parties involved in a communication can trust each other's identities, which is crucial for secure data exchanges over networks.

While firewalls focus on controlling incoming and outgoing network traffic based on predetermined security rules, and active directory provides a means to manage computers and other devices on a network but does not specifically handle identity verification through certificates, intrusion detection systems primarily monitor network traffic for suspicious activity and do not involve the use of certificates for identity verification. PKI is the definitive solution for environments where secure identity confirmation through cryptographic means is essential.