Answer:
B. Resource pooling.
Step-by-step explanation:
-On-demand means that clients can get the service when and where they need it.
-Resource pooling refers to grouping resources to provide the service to different customers in a way in which the resources are assigned according to the demand of each client.
-Measured Service is when a company monitors the service that is providing for different purposes like billing.
-Rapid Elasticity is the ability that the company providing the service has to modify the resources according to the demand.
According to the definitions, the answer is that the cloud computing element that gives a service provider the ability to dynamically allocate shared physical resources to multiple customers based on each customer's usage demand is resource pooling.