The correct answer is Virtualization.

Virtualization provides the hardware and software abstraction layer that allows cloud providers to run many isolated environments on the same physical servers. This abstraction makes it possible to provision virtual machines and containers quickly, to isolate tenants and workloads, and to apply resource controls and live migration features that modern cloud platforms require.

Object storage is an important cloud service for storing unstructured data, but it is an application level capability and not the underlying technology that enables the creation of isolated compute instances.

Multitenancy is a design goal and outcome of cloud platforms because they serve multiple customers on shared infrastructure, but it is enabled by virtualization rather than being the foundational enabling technology itself.

Resource pooling describes how cloud providers share and allocate resources across customers, but it is a characteristic of cloud operations rather than the core technology that makes those operations possible.

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