Which of these tools provides a a solution that allows a user to have their calendar on both their smart phone, desktop, and be accessible through a web portal (such as outlook web access) all in real time?
Cloud synchronization is the correct answer, it allows a user to share information (contacts, calendars, photos, etc.) between devices with an internet connection. POP3 is a protocol for email, tethering is for sharing an internet connection from your mobile device to another device and VPN is for a way to remotely connect to a network.