Final Answer:
In order for users to synchronize between on-premises Exchange and Office 365 via Active Directory Connect, c) UserPrincipalName and proxyAddresses attributes must be unique.
Step-by-step explanation:
For users to synchronize between on-premises Exchange and Office 365 using Active Directory Connect, the two attributes that must be unique are UserPrincipalName (UPN) and proxyAddresses. The UserPrincipalName serves as the user's identity within the Active Directory domain, while proxyAddresses include the user's email addresses.
Ensuring the uniqueness of these attributes is crucial for proper synchronization, avoiding conflicts, and maintaining a seamless experience for users moving between on-premises and cloud environments. Options A, B, and D do not represent the correct attributes needed for synchronization.
Option C is the answer.