Which of the following tools would be the best choice for achieving compliance with PCI Requirement 11?

TrueCrypt is a disk encryption utility relevant to PCI Requirement 3 (protect stored cardholder data), not Requirement 11, which focuses on active vulnerability scanning and penetration testing.

Sub7 is a Remote Access Trojan used as malicious attack software and has no legitimate role in any PCI DSS compliance requirement.

Nessus by Tenable is a widely used vulnerability scanner that performs the internal and external network scans required by PCI DSS Requirement 11.2, which calls for quarterly vulnerability assessments. The PCI Security Standards Council recognizes approved scanning vendors (ASVs) and Nessus is widely used in that capacity to identify missing patches, misconfigurations, and known CVEs. Its direct alignment with automated vulnerability discovery makes it the best tool match for satisfying Requirement 11 compliance.

ClamWin is an open-source antivirus tool relevant to PCI Requirement 5 (protect all systems against malware), not Requirement 11, which specifically mandates vulnerability scanning and security testing.