Final answer:
The expenditure model for migrating virtual machines to an Azure pay-as-you-go subscription is Operational Expenditure (OpEx), which is based on resource consumption over time.
Step-by-step explanation:
When planning to migrate all virtual machines to an Azure pay-as-you-go subscription, the expenditure model you should identify is Operational Expenditure (OpEx). This model is associated with ongoing costs for services and products. Unlike Capital Expenditure (CapEx), which involves upfront investment in physical assets, OpEx aligns with the pay-as-you-go approach, charging based on the consumption of services over time, such as the resources used within Azure.