To ensure that a PaymentForm controller uses the HTTPS protocol in Salesforce B2C Commerce, the correct approach is to use the guard.ensure function with the 'https' argument along with 'post' and 'loggedIn'. Option D, exports.PaymentForm = guard.ensure(['https', 'post', 'loggedIn'], handlePaymentForm);, correctly specifies that the PaymentForm handler should be accessible only via HTTPS, ensuring secure communication over the network. The guard.ensure method is a part of the application security framework designed to enforce certain conditions like HTTP methods and secure protocols before a controller action is executed. This is documented in the Salesforce B2C Commerce security best practices, where specifying 'https' ensures that any data transmitted during the payment process is encrypted and secure.