Your company is running an application on an on-premise web server. You need to provide a set of remote users access to the application without providing VPN access. You need to create a solution to provide access by using the least amount of administrative effort.

Answer options:

A.Migrate the Web server to Azure as IaS
B.Migrate the application to Azure App Service
C.Install and register Azure AD Application Proxy
D.Install AD Connect on a Domain Controller

Correct Answer: C 
You should download and install the Azure AD Application Proxy agent on your web server. The service enables you to connect internal applications to external public HTTP/HTTPS URL endpoints in Azure. 
Download the agent from Azure Active Directory – Application proxy: 

Then register and enable it in Azure AD: 

Confirm that your web server has shown up in Azure AD with status “Active”, and then navigate to Azure AD - Enterprise Application 
Select “New application” and add an on-premises application. 

Provide the needed information of your on-premise Web application and select “Add”. 

Verify that your on-premises application is defined in Azure AD.Next you add provide user access to the application by selecting 
“Users and Groups” – “Add users” and add the set of users that should have access to the application. 

Remote access has now been successfully assigned. 
As a best practice you might also consider enabling Single sign-on for the application, though this is out of scope of this scenario. 

Option A is incorrect. Migrating the web server to Azure as infrastructure as a service would provide access to remote users, but it involves more planning and configuration than using an Application Proxy. We are to use the least amount of administrative effort, therefore this is not the correct answer.
Option B is incorrect. Migrating the on-premise application to Azure App Service would provide access to remote users, and this would require less effort than
Option A. But it involves more planning and configuration than using an Application Proxy. We are to use the least amount of administrative effort, therefore this is not the correct answer.
Option D is incorrect. Installing AD Connect will not provide access to your on-premise application as this is a service to synchronization of users and groups between local and Azure AD. 
Reference:
To know more about providing remote access through application proxy, please refer to the link below: 

https://docs.microsoft.com/en-us/azure/active-directory/app-proxy/application-proxy