To ensure that traffic is encrypted in transit at all times between the users and the backend, the company must implement HTTPS for both the CloudFront distribution and the Application Load Balancer (ALB). This involves creating and using SSL/TLS certificates for both the CloudFront custom domain and the ALB domain. The correct steps include: 1) Creating a certificate for the CloudFront custom domain (service.example.com) using AWS Certificate Manager (ACM) and configuring CloudFront to use this certificate, while also setting the default behavior to redirect HTTP to HTTPS (Option B). 2) Creating a certificate for the ALB domain (service-alb.example.com) using AWS Certificate Manager (ACM), adding a new HTTPS listener on the ALB that uses this certificate, and modifying the CloudFront origin to use the HTTPS protocol only, while deleting the HTTP listener on the ALB (Option E). 3) Ensuring that the backend EC2 instances are configured to use HTTPS by creating a certificate for them, specifying the instance target type during the creation of a new target group that uses the HTTPS protocol, and attaching the existing Auto Scaling group to this new target group (Option C). These steps ensure end-to-end encryption of traffic.