Final answer:
The appropriate use of a DoD Public Key Infrastructure (PKI) token is for accessing confidential military data, as it is designed for security practices within military contexts and not for personal or commercial use.
Step-by-step explanation:
Among the listed options, the appropriate use of a DoD Public Key Infrastructure (PKI) token is for accessing confidential military data. PKI tokens are typically used to securely authenticate individuals and devices in various security domains. Given the DoD or Department of Defense, context, such a token would be intended for actions directly related to military functions, such as securing communications and data access.
Logging into social media accounts, online shopping, and personal email encryption, while all activities that might require secure authentication, would not usually necessitate the high security level of a DoD PKI token, and thus are not appropriate uses of this kind of token. The DoD PKI token use is specific to defending against threats to national security, safeguarding sensitive military communications, and controlling access to classified or mission-critical information.