An application consists of a set of virtual machines hosted in a Virtual Network. In a month, the application seems to have a load of around 20% for 3 weeks. During the last week, the load on the application reaches 80%. Which of the following benefits of Azure Cloud would ensure the underlying application infrastructure`s cost and efficiency?

Answer options:

A.High availability
B.Elasticity
C.Disaster recovery
D.Fault tolerance

Answer – B 
Here the concept refers to Elasticity. In this use case, you could define a Virtual Machine Scale set in Azure.
You could define an initial set of Virtual Machines as part of the scale set that would run for the first 3 weeks.
You can then add a scaling policy to add more Virtual Machines to support the application during the last week.
You can then add a scaling policy to remove the extra Virtual Machines at the end of last week to save on costs.
All other options are incorrect since they all refer to the availability of the infrastructure. 
For more information on Virtual Machine scale sets, please visit the below URL- 

https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/overview