Final answer:
The root CA is responsible for certifying its own public key pair through a process known as self-signing, which establishes the root of trust in the PKI hierarchy.
Step-by-step explanation:
In the context of Public Key Infrastructure (PKI), the entity responsible for certifying the public key pair of a root Certificate Authority (CA) is typically the root CA itself. The root CA is at the top of the certification hierarchy and is therefore self-signed. This means that the root CA uses its own private key to create a digital signature that validates its public key. Trust in the root CA is essential, as all certificates issued by the root and subordinate CAs derive their trust from the root CA's integrity and security practices.