John Has Subscribed to a Cloud-Based Service to Synchronize Data

Multiple Choice

John has subscribed to a cloud-based service to synchronize data between his smartphone, tablet, and PC. Before allowing the data to be synchronized, this service uses a process in which the cloud service and the client on the smartphone will verify the security of all devices' security certificates.
What type of security does the cloud-base service employ?

A) Mutual authentication
B) VPN
C) IMAP
D) Tethering