Which EC2 instance purchasing option should the company use to meet these requirements?

Explanations:

Dedicated Instances provide isolation at the hardware level but do not provide the necessary capacity to bring existing licenses. They are typically used for compliance or regulatory requirements, not for license portability.

Reserved Instances allow for capacity reservation for a specific instance type and size but do not provide the ability to bring existing licenses. They are cost-saving options rather than options for license mobility.

Dedicated Hosts allow customers to use their existing Microsoft SQL Server licenses under the License Mobility program. This option provides a physical server dedicated to a single customer, which is ideal for bringing existing licenses.

Spot Instances are cost-effective but are not suitable for running critical applications that require consistent performance. They do not provide any support for bringing existing licenses.