The GREATEST advantage of using a virtual private network (VPN) over dedicated circuits and dial-in servers is that it is more cost effective. This is because a VPN is a technology that creates a secure and encrypted connection between a client and a server over an existing public network, such as the Internet. A VPN reduces the cost of establishing and maintaining a secure communication channel, as it does not require any additional hardware, software, or infrastructure, unlike dedicated circuits and dial-in servers, which require dedicated lines, modems, routers, switches, etc. The other options are not the greatest advantage of using a VPN over dedicated circuits and dial-in servers, because they either involve security (A), reliability (B), or speed C aspects that may not be significantly different or better than dedicated circuits and dial-in servers.