The correct answer is C, which involves spreading EC2 instances across more than one Availability Zone. This approach enhances the reliability of the web application by providing redundancy and fault tolerance. In the event of a failure in one Availability Zone, the application can continue to operate in another, ensuring high availability. Launching large instances in the same Availability Zone (A) does not provide the necessary redundancy. Spreading instances across different security groups (B) is not directly related to the application's reliability. Using an AMI from AWS Marketplace (D) is a method for deploying instances but does not inherently increase reliability.