Answer:c)A digital document that is used to verify the owner's identity
Explanation: Certificate is termed as the digital document that helps in exchanging information or data with protection over the internet.It is used for the maintenance of authentication of user .They are provided with he username and password for the security which can be accessed by the authorized party only.
It verifies the the message sent is by the authorized user and the receiver the decrypts the message.Other options are incorrect because it does not encrypts the data rather has encrypted password and does not certify or change the key's strength .Thus, the correct option is option(c).