Explanation:
One of the major changes that you will face when you move from on-premises cloud to the public cloud is the switch from capital expenditure (buying hardware) to operating expenditure (paying for service as you use it).
Box 1: No
With the pay-as-go model, you pay for services as you use them. This is Opex (Operational Expenditure), not CapEx (Captial Expenditure). CapEx is where you pay for something upfront.
For example, buying a new physical server.
Box 2: No
A reserved instance is where you pay upfront for the use of a virtual machine for a period of time (1 or 3 years). This can save you money as you receive a discount on the cost of a VM if you pay upfront for a reserved instance. However, as this is an upfront payment, it will be classed as CapEx, not OpEx.
Box 3: Yes
Deploying your own datacenter is an example of CapEx. This is because you need to purchase all the infrastructure upfront before you can use it.
Reference:
https://docs.microsoft.com/en-us/azure/architecture/cloud-adoption/appendix/azure-scaffold