HTTPS (Hypertext Transfer Protocol Secure) is the correct answer.

- The Hypertext Transfer Protocol (HTTP) is a protocol used for communication between a client and a server over a network. However, it is not secure because the data exchanged between the client and server is not encrypted, making it vulnerable to interception and tampering.
- FTP (File Transfer Protocol) is a protocol used for transferring files between a client and a server. Similar to HTTP, FTP is not secure and does not provide encryption for the data being transferred.
- TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of protocols that govern the transmission of data over the Internet. While TCP/IP is essential for communication over the Internet, it does not specifically address security.
- SMTP (Simple Mail Transfer Protocol) is a protocol used for sending emails between servers. Like HTTP and FTP, SMTP does not provide encryption for the email contents.

HTTPS, on the other hand, is a combination of HTTP and SSL/TLS (Secure Sockets Layer/Transport Layer Security) protocols that adds an extra layer of security to HTTP. Here's how it works:

1. Encryption: HTTPS uses SSL/TLS to encrypt the data transmitted between the client and the server. Encryption ensures that the data is scrambled and cannot be easily deciphered if intercepted by unauthorized parties.

2. Authentication: SSL/TLS also provides authentication, which ensures that the client is communicating with the intended server and not an impostor. This is achieved through the use of digital certificates issued by trusted Certificate Authorities.

3. Data Integrity: HTTPS ensures data integrity by using hash functions that verify the integrity of the transmitted data. If any tampering is detected, the connection is terminated.

4. Port: HTTPS typically uses port 443 instead of the default HTTP port 80. This allows the server to distinguish between HTTP and HTTPS requests.

In summary, HTTPS is used for secure communication over the Internet by encrypting data, providing authentication, and ensuring data integrity. It is commonly used for secure transactions, such as online banking, e-commerce, and sensitive data transfers.