The correct answer is D, Cost optimization. The AWS Well-Architected Framework's Cost Optimization pillar focuses on using AWS resources efficiently and cost-effectively. It encourages organizations to select the right types and sizes of resources, leverage elasticity to scale resources based on demand, and use automation to optimize resource utilization. The company's strategy to pay only for the resources it uses and to dynamically scale its resource usage aligns with the principles of cost optimization, which aims to minimize costs while achieving the desired business outcomes without sacrificing performance, security, or reliability.