The correct answer is A: Use AWS Storage Gateway for files to store and process the video content. This option is the most cost-effective as AWS Storage Gateway enables integration of on-premises software appliances with cloud-based storage to provide seamless and secure integration between an organization's on-premises IT environment and AWS's storage infrastructure. This reduces the cost associated with using Amazon EFS for direct storage, which can become expensive at scale. Using Storage Gateway for files takes advantage of AWS's lower-cost storage options while still allowing the required accessibility for processing tasks.