Explanation
Azure customers with an Azure Enterprise Agreement (EA), Microsoft Customer Agreement (MCA), or Microsoft Partner Agreement (MPA) can use Azure Cost Management.
Cost management is the process of effectively planning and controlling costs involved in your business. Cost management tasks are normally performed by finance, management, and app teams. Azure Cost Management
+ Billing helps organizations plan with cost in mind. It also helps to analyze costs effectively and take action to optimize cloud spending.
References:
https://docs.microsoft.com/en-gb/azure/cost-management/overview-cost-mgt

Explanation

Azure AD join only applies to Windows 10 devices.
References:
https://docs.microsoft.com/en-us/azure/active-directory-domain-services/manage-group-policy
https://docs.microsoft.com/en-us/azure/active-directory/devices/azureadjoin-plan

Explanation
Elasticity in this case is the ability to provide additional compute resource when needed and reduce the compute resource when not needed to reduce costs. Autoscaling is an example of elasticity.
Elastic computing is the ability to quickly expand or decrease computer processing, memory and storage resources to meet changing demands without worrying about capacity planning and engineering for peak usage. Typically controlled by system monitoring tools, elastic computing matches the amount of resources allocated to the amount of resources actually needed without disrupting operations. With cloud elasticity, a company avoids paying for unused capacity or idle resources and doesn't have to worry about investing in the purchase or maintenance of additional resources and equipment.
https://azure.microsoft.com/en-gb/overview/what-is-elastic-computing/