Which of the following issues is not addressed by digital signatures?

A digital signature directly addresses both confidentiality and integrity of the CIA triad. It does not directly address availability, which is what denial-of-service attacks.

The other answers are not correct because:

"nonrepudiation" is not correct because a digital signature can provide for nonrepudiation.

"authentication" is not correct because a digital signature can be used as an authentication mechanism

"data integrity" is not correct because a digital signature does verify data integrity (as part of nonrepudiation)

References:

Official ISC2 Guide page: 227 & 265

All in One Third Edition page: 648