Federation Services is the correct answer because it enables the secure sharing of identity information between disparate security domains. This means that the users can use their on-premises Active Directory credentials to access cloud-based VDI, as it will create a trust relationship between the on-premises directory and the cloud provider. On the other hand, IdP integration is useful, but it refers to the integration of a third-party Identity Provider, which is not the scenario described. Certificate services are part of Public Key Infrastructure (PKI) and primarily deal with the creation, management, and distribution of digital certificates, which is not directly related to the use of existing on-premises credentials. Lastly, the Multifactor Authentication solution adds an extra layer of security but does not specifically address the use of on-premises credentials in the cloud VDI environment.