What must a QSA provide to prove they are maintaining their qualification?

A QSA must provide proof of security training to demonstrate they are maintaining their qualification. This training is essential because it ensures that the assessor stays up to date with current practices, regulatory requirements, and the evolving landscape of security threats. Continuous education and proof of training help maintain the integrity and credibility of the QSA's role in assessing compliance with PCI DSS standards.

In the context of the other options, while complete assessments and client feedback reports are important elements of the QSA's work and may indirectly reflect their capabilities, they do not serve as direct evidence of ongoing qualification maintenance. Utility bills are unrelated to the qualifications or capabilities of a QSA, as they do not provide any relevant information about the assessors' professional training or expertise. Therefore, proof of security training is the most pertinent requirement to uphold a QSA's qualification status.