To manage a process where users can order a new iPhone only after receiving two levels of approval, with automatic notifications at each approval level, the most suitable feature to use in ServiceNow would be:

E. Flows

Using the Flow Designer in ServiceNow, you can create complex multi-level approval workflows that include conditions for approvals and actions for sending notifications. This tool allows you to automate the process end-to-end, ensuring that each step is followed and that the necessary parties are informed at each stage of the approval process. It’s a powerful feature for managing approvals and notifications efficiently within ServiceNow.