A: Application Proxy is a feature of Azure AD that enables users to access on-premises web applications from a remote client. Application Proxy includes both the Application Proxy service which runs in the cloud, and the Application Proxy connector which runs on an on-premises server.
You can configure single sign-on to an Application Proxy application.
C: Microsoft recommends using Application Proxy with pre-authentication and Conditional Access policies for remote access from the internet. An approach to provide Conditional Access for intranet use is to modernize applications so they can directly authenticate with AAD.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/app-proxy/application-proxy-config-sso-how-to
https://docs.microsoft.com/en-us/azure/active-directory/app-proxy/application-proxy-deployment-plan