Reference:
https://azure.microsoft.com/en-us/support/legal/sla/summary/

To migrate to Azure and decommission the on-premises data center, you would need to create the 5,000 user accounts in Azure Active Directory. The easy way to do this is to sync all the Active Directory user accounts to Azure Active Directory (Azure AD). You can even sync their passwords to further minimize the impact on users.
The tool you would use to sync the accounts is Azure AD Connect. The Azure Active Directory Connect synchronization services (Azure AD Connect sync) is a main component of Azure AD Connect. It takes care of all the operations that are related to synchronize identity data between your on-premises environment and Azure AD.
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-whatis

Reference:
https://azure.microsoft.com/en-us/free/
https://azure.microsoft.com/en-us/free/free-account-faq/
https://docs.microsoft.com/en-us/azure/billing/billing-avoid-charges-free-account
https://azure.microsoft.com/en-us/free/free-account-faq/
https://docs.microsoft.com/en-us/azure/billing/billing-avoid-charges-free-account

There are different payment options in Azure including pay-as-you-go (PAYG), Enterprise Agreement (EA), and Microsoft Customer Agreement (MCA) accounts.
Your Azure costs are 'per subscription'. You are charged monthly for all resources in a subscription. Therefore, to use different payment options per department, you will need to create a separate subscription per department. You can create multiple subscriptions in a single Azure Active Directory tenant.
Incorrect Answers:
A: A reservation is where you commit to a resource (for example a virtual machine) for one or three years. This gives you a discounted price on the resource for the reservation period. Reservations do not provide a way to use different payment options per department.
C: A resource group is a logical container for Azure resources. You can view the total cost of all the resources in a resource group. However, resource groups do not provide a way to use different payment options per department.
D: A container instance is an Azure resource used to run an application. Container instances do not provide a way to use different payment options per department.
Reference:
https://docs.microsoft.com/en-us/azure/cost-management-billing/manage/create-subscription