Final answer:
To prevent a user on vacation or leave from logging into Office 365 without deleting their account, you need to use the 'Block-UserAccess' command.
Step-by-step explanation:
The correct answer to prevent a user on vacation or leave from logging into Office 365 without temporarily deleting their account is c) Block-UserAccess -User.
This command will block the user's access to Office 365 while preserving their account. By blocking their access, the user will not be able to log in or access any Office 365 services until the access is unblocked.