The defining characteristic of an Integration-Platform-as-a-Service (IPaaS) is that it is cloud-based. The "as a Service" part of the acronym signifies a service delivery model where the platform is hosted, managed, and maintained by a third-party vendor and delivered to customers over the internet. This abstracts the underlying infrastructure, allowing users to focus on developing, deploying, and managing integrations without the overhead of maintaining the platform hardware or software. While an IPaaS may support various development styles (code-first, no-code) and deployment models (hybrid), its fundamental nature is a cloud-delivered service.

A. Code-first: This is a development methodology, not a platform delivery model. Many IPaaS platforms are designed to be low-code first to increase accessibility.

C. On-premises: This is the opposite of the "as a Service" model. On-premises solutions require the customer to host and manage the infrastructure and software.

D. No-code: This is a feature of many IPaaS offerings to empower non-developers, but it is not a defining requirement, as they also support complex, code-intensive integrations.

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1. MuleSoft Official Website, "What is an iPaaS (Integration Platform as a Service)?": The official definition states, "iPaaS is a cloud-based platform that connects various applications, data, and systems... Because it is cloud-based, iPaaS provides greater flexibility and scalability than traditional, on-premise integration solutions." (MuleSoft, Inc., "What is an iPaaS?", Section 1, Paragraph 1).

2. MuleSoft Documentation, "Anypoint Platform Overview": The documentation describes the core components of the Anypoint Platform, such as the control plane (e.g., Design Center, Management Center), which are hosted by MuleSoft in the cloud. This architecture is central to the IPaaS model. (MuleSoft Docs, "Anypoint Platform", "Anypoint Platform Overview" section).