Which XML-based protocol is commonly used by credit card companies to enhance the security of online transactions?

3-D Secure is the correct answer because it is specifically designed as a security protocol for online credit card transactions. It adds an additional layer of authentication during the payment process, requiring shoppers to provide a password or a verification code that is known only to them and their card issuer, thus reducing the risk of fraud.

This protocol is utilized by various credit card companies under different brand names, such as Verified by Visa, MasterCard SecureCode, and American Express SafeKey. The presence of 3-D Secure enhances customer confidence in the security of their online purchases and allows merchants to better manage chargebacks and fraud liability.

Other protocols and technologies, while essential for secure transactions, serve different roles. Secure Socket Layer (SSL) and its successor, Transport Layer Security (TLS), provide encryption for data in transit but do not specifically add a layer of authentication for cardholders. Hypertext Transfer Protocol Secure (HTTPS) also focuses on encryption and securing communications over a computer network but does not offer the consumer authentication feature unique to 3-D Secure. Thus, 3-D Secure stands out as the protocol dedicated to enhancing security in online credit card transactions.