Final answer:
Device identity is the first possible authentication method for Things.
Step-by-step explanation:
The first possible authentication method for Things is Device identity (C). Device identity authentication involves verifying the identity of the device connecting to a network. This can be done by associating each device with a unique identifier, such as a MAC address or an IP address. By authenticating the device's identity, it can be ensured that only authorized devices are allowed to connect to the network.