Which of these is NOT one of the six pillars of the AWS Well-Architected Framework?

"Scalability" is not a pillar. The six pillars are:
• Operational Excellence
• Security
• Reliability
• Performance Efficiency
• Cost Optimization
• Sustainability

Scalability matters, but the framework covers it inside Reliability and Performance Efficiency. It is not a pillar of its own. "Sustainability" is a real pillar: AWS added it as the sixth pillar in 2021.