This question asks you to match specific Microsoft services to their corresponding cloud service models. Power Apps is a platform for building low-code applications, placing it in the PaaS category. PaaS provides a development and deployment environment, abstracting the underlying infrastructure. Microsoft 365 apps (like Word or Excel) are ready-to-use software applications accessed over the internet, which is the definition of SaaS. Finally, Azure virtual machines represent the provisioning of fundamental computing resources like virtual servers and storage, which is the core offering of IaaS.

Microsoft Power Apps as PaaS: Microsoft Docs, "What is Power Apps?", section "Understanding the cloud service model" (latest version). This documentation explicitly defines Power Apps as a platform for application development, which aligns with the PaaS model.

Microsoft 365 apps as SaaS: Microsoft Docs, "Microsoft 365 Licensing and Service Descriptions", under the section "Core Services". This resource describes Microsoft 365 apps as subscription-based software services delivered over the internet, which is the standard definition of SaaS.

Azure virtual machines as IaaS: Microsoft Docs, "Introduction to Azure Virtual Machines", section "What is an IaaS offering?". This document identifies Azure virtual machines as providing basic computing infrastructure such as servers, storage, and networking, which is the fundamental definition of IaaS.

Academic Source on Cloud Models: Mell, Peter, and Tim Grance. "The NIST Definition of Cloud Computing." National Institute of Standards and Technology (NIST) Special Publication 800-145, September 2011, p. 2-3. This seminal publication defines the three cloud service models: SaaS, PaaS, and IaaS, providing the foundational definitions used by the industry, which helps to categorize the given services accurately.