A company is planning to set up a series of Azure SQL Databases. Which of the following feature can they use to reduce the overall cost if the databases have unpredictable usage demands?

Answer options:

A.Azure Reservation
B.Elastic Pools
C.Single databases
D.Clustered databases

Answer – B 
You can use Elastic pools to make more efficient use of the costs when having databases with unpredictable usage demands. 
The Microsoft documentation mentions the following. 

Since this is clear from the documentation, all other options are incorrect. 
For more information on Elastic pools, please visit the below URL- 

https://docs.microsoft.com/en-us/azure/sql-database/sql-database-elastic-pool