Which protocol is used to secure web traffic?

Correct answer: HTTPS

Option C is correct because HTTPS (Hypertext Transfer Protocol Secure) uses TLS to encrypt web traffic between clients and servers, ensuring confidentiality, integrity, and authentication for web communications. Option A is wrong because FTP (File Transfer Protocol) is used for file transfers and transmits data in cleartext without encryption. Option B is incorrect because HTTP is the unencrypted version of the web protocol and does not provide any security for data in transit. Option D is wrong because SMTP is an email transmission protocol and is not used to secure general web traffic, though it can be secured separately with TLS.