To answer this question, the user needs to have knowledge about cookie flags and their purposes.
The Secure flag is used to ensure that cookies are only transmitted over an encrypted (HTTPS) connection. So, it is the flag that will prevent cookies from being transmitted over a non-secure channel.
Option A: Secure is correct.
Option B: Domain is used to specify the domain name to which the cookie should be sent. This flag is not related to preventing transmission over non-secure channels.
Option C: Expires is used to set an expiration time for the cookie. This flag is not related to preventing transmission over non-secure channels.