An Amazon EC2 Auto Scaling group with a target tracking scaling policy is the best solution for the described use case as it allows automatic adjustment of the number of EC2 instances in response to the load on the application. Target tracking scaling policies adjust the capacity to maintain a specific target for a selected metric such as average CPU utilization or number of requests per target. This is suitable for handling unpredictable, spikey traffic because it provides elasticity by automatically increasing or decreasing the number of EC2 instances as required to meet the target. The other options, such as fixed-size EC2 groups or an Application Load Balancer without Auto Scaling, do not offer the same level of elasticity and cannot automatically adjust the compute capacity in response to varying loads.