Which combination of steps will meet these requirements?

Explanations:

While replacing the EC2 instance with a larger compute optimized instance may provide better performance, it does not offer high availability or scalability as it still relies on a single instance. This solution does not address the issue of demand exceeding capacity effectively.

Configuring Amazon EC2 Auto Scaling with multiple Availability Zones ensures that the application can automatically adjust the number of EC2 instances based on traffic demand and remain highly available. Distributing instances across multiple Availability Zones also protects against zone failures.

Configuring a NAT gateway is primarily for allowing instances in a private subnet to access the internet. It does not directly address the requirement for high availability and scalability of the web application hosted on a single EC2 instance in a public subnet.

Similar to option A, replacing the EC2 instance with a larger memory optimized instance may enhance performance but does not provide a solution for high availability or scalability. It still relies on a single instance which is a single point of failure.

Configuring an Application Load Balancer in a public subnet will allow for the distribution of incoming web traffic across multiple EC2 instances, enabling better handling of increased traffic and providing high availability. The load balancer also integrates with Auto Scaling for dynamic scaling based on demand.