The correct answer is D. Enabling HTTPS-Only Mode, because HTTPS ensures that all communication between the browser and websites is encrypted using TLS (Transport Layer Security). This protects sensitive information such as usernames, passwords, and payment details from interception.

According to the Quentin Docter – CompTIA A+ Complete Study Guide, HTTPS encrypts data in transit, preventing attackers from eavesdropping or modifying communications. HTTPS-only modes in modern browsers block or warn users when a site does not support secure connections.

The Travis Everett & Andrew Hutz – All-in-One Exam Guide emphasizes that HTTPS is essential for protecting authentication credentials and financial information, especially on public or untrusted networks. Enforcing HTTPS-only connections ensures users cannot accidentally submit data over unencrypted HTTP.

The Mike Meyers / Mark Soper Lab Manual clarifies that disabling cookies or JavaScript may break website functionality but does not guarantee encryption. Pop-up blockers address nuisance behavior, not data security.

Because the objective is to encrypt browser communications and protect sensitive data, enabling HTTPS-Only Mode is the most effective and direct solution, making D the correct answer.