Amazon EC2 Auto Scaling groups are designed to automatically adjust the number of EC2 instances in response to changes in demand for your application. This process, known as scaling in and out, helps maintain the desired capacity to handle the load efficiently and cost-effectively. While option C suggests scaling the size of instances, which is also a feature of EC2 Auto Scaling, the question specifically asks about scaling 'capacity,' which typically refers to the number of instances rather than their size. Therefore, the correct answer is A, as it directly addresses the scaling of the number of EC2 instances based on demand.