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Solution

In cloud computing, pooling and sharing of resources means multiple users can use shared computing resources like CPU, memory, storage and network.

Virtualization is the main concept that helps in pooling and sharing of resources.

Using virtualization, one physical machine can be divided into multiple virtual machines.

So, many users can share the same physical resources.

Virtual Memory is an operating system concept, not mainly a cloud computing concept for resource pooling.

Service means providing facilities like $IaaS$, $PaaS$ and $SaaS$, but the direct concept related to pooling and sharing is virtualization.

Therefore, only $(C)$ Virtualization is correct.