This is a part of a question set containing 2 questions: 
You are responsible for the Office 365 security in your organization. You want to block legacy authentication to Azure AD as these protocols do not support MFA. First you need to know if your users have apps that use legacy authentication. 
Where should you look? 

Answer options:

A.Azure Active Directory – Azure AD Connect
B.Azure Active Directory – Logs
C.Azure Active Directory – Sign-ins
D.Azure Active Directory – Usage & Insights

Correct Answer: C 
You can determine who is using legacy authentication by navigating to Azure Portal-> Azure Active Directory -> Sign-ins. 
Choose the Client App Column (add the client app column if it is not shown). Filter the column and check all legacy authentication clients options. 
Filter by status Success: 

Option A is incorrect. Azure AD Connect allows you to manage sync configurations in the cloud, and syncing Active Directory users and groups. 
Option B is incorrect. Azure Active Directory Logs uses a log analytics workspace to query selected metrics and data. You cannot view the authentication methods from Logs. 
Option D is incorrect. Azure Active Directory – Usage& Insights provides information about your most active applications and sign in success rate.
Reference: 
To know more about legacy authentication, please refer to the link below: 

https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/block-legacy-authentication