Which feature should be employed to improve payment security during online transactions?

Requiring 3D Secure enhances payment security during online transactions by adding an additional layer of authentication. This protocol mandates that the cardholder undergo a verification process, typically through a one-time password or biometric identification, provided by their card-issuing bank. This helps to confirm that the person initiating the transaction is indeed the legitimate cardholder, thereby reducing the risk of fraud and chargebacks.

While email verification adds a level of security, it is not as robust as 3D Secure, which is specifically designed for secure transactions. Using plaintext passwords jeopardizes security by making it easier for unauthorized individuals to obtain sensitive information. Similarly, offering limited payment options does not directly enhance transaction security; rather, it may reduce customer satisfaction by restricting payment choices without addressing security vulnerabilities. Therefore, requiring 3D Secure is the most effective method among the choices given to ensure secure and safe online transactions.