Elasticity is the correct answer because it refers to the AWS Cloud's ability to automatically scale computing resources up or down based on demand. This characteristic allows users to:

• Eliminate underutilized CPU capacity by scaling down resources when they're not needed
• Scale up resources during peak usage periods
• Pay only for the resources actually consumed
• Automatically adjust capacity to match workload requirements

Why other options are incorrect:

• Agility: Refers to the speed and ease of deploying resources, not capacity optimization
• Reliability: Focuses on service availability and fault tolerance
• Durability: Concerns data persistence and protection against data loss

Elasticity is a core benefit of cloud computing that enables cost optimization by preventing over-provisioning and eliminating wasted resources.