The correct answer is A: Utilize AWS Snowball. AWS Snowball is designed for transferring large amounts of data to AWS when network connections are limited or unreliable. Given the constraints of a 15Mbps bandwidth and a 70% utilization cap, using AWS Snowball will help migrate the 20 TB of data efficiently within the 30-day timeframe. Other options, such as DataSync, VPN, and S3 Transfer Acceleration, would be less effective given the network limitations.