High availability and elastic scalaility for web servers?

Use EC2 Auto Scaling and an ALB across Multi AZs

Low-latency connections over UDP to a pool of instances running a gaming application?

Use NLB with a UDP listener

Clients need a whitelist static IP addresses for a highly available load balanced application in an AWS Region?

use an NLB and create static IP addresses in each AZ.

Application on EC2 in an Auto Scaling group requires disaster recovery across Region?

Create an ASG in another Region with the capacity set to 0. Take snapshots and copy them across region (Lambda or DLM)

Application on EC2 must scal in larger increments if a big increase in traffic occurs, compared to small increases in traffic?

Use Auto Scaling with a Step scaling policy and configure a larger capacity increase.

Need to Scale EC2 instances behind an ALB based on the number of requrests completed by each instance?

Configure a target tracking policy using the ALBRequestCountPerTarget metric.

Need to run a large batch computing job at the lowest cost. Must be managed. Nodes can pick up where others left off in case of interruption?

Use a managed AWS Batch job and use EC2 Spot instances.

A tightly coupled High Performance Computing (HPC) workload requries low-latency between nodes and optimum network performance?

Launch EC2 instances in a signle AZ in a cluster placement group and use an Elastic Fabric Adapter (EFA)

Application must startup quickly when launched by ASG but requires app dependencies and code to be installed?

Create an AMI that includes the application depnedencies and code.

Application runs on EC2 behind an ALB. Once authenticated users should not need to reauthenticate if an instance fails?

Enable Sticky session for the target group or alternatively use a session store such as DynamoDB

LOB Application receives weekly burst of traffic and must scale for short periods - need thte most cost-effective solution?

Use reserved instsances for minimum required workload and then use Spot instances for the bursts in traffic.