For a customer-facing banking application, the most critical issue is protecting sensitive data while it is transmitted between the customer and the web application. Account numbers, credentials, balances, and other confidential information can be intercepted over networks if communications are not encrypted. Encryption in transit, usually through secure protocols such as TLS, protects confidentiality and integrity during transmission. Option A is useful because logs may contain sensitive information, but it is not the most important control for customer data moving across the internet. Option B may be important for data at rest, but the question focuses on a web-based customer application and submitted data. Option C is relevant to physical security, but it does not address interception risk during online communication. Therefore, Option D is the strongest answer.