Final answer:
To address MFA issues with Office 365, you should open a Microsoft support ticket under 'Multi-Factor Authentication' to get specialized assistance.
Step-by-step explanation:
If you're experiencing MFA issues with Office 365 and need to open a support ticket with Microsoft, the correct category to choose would be 'a) Multi-Factor Authentication'. This category is specifically designated for problems related to the multi-factor authentication system. Opening a ticket under this category will ensure that you receive assistance from technicians who specialize in MFA and security issues, providing you with the most efficient and accurate support for your problem.