Apki (public key infrastructure) is an arrangement that binds public keys with respective user identities by means of a

Certificate authority

Explanation:

A PKI is stand for the public key infrastructure and by using the PKI it binding all the public keys by their specific user identity in the form of (CA) certificate authority.

The main role of the public key infrastructure is that it helps in binding the public keys by using the registration authority method and it is responsible for ensuring all the registration done efficiently and correctly.

 Therefore, Certificate authority is the correct answer.  

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