The correct answer is SaaS (Software as a Service). According to the CompTIA Network+ N10-009 objectives, SaaS is the most cost-efficient cloud model for hosting common business applications such as email, collaboration tools, and productivity suites. With SaaS, the cloud provider manages all infrastructure, operating systems, application software, security updates, and maintenance, significantly reducing administrative overhead and operational costs.

Email services are a prime example of workloads that benefit from SaaS solutions such as Microsoft 365 or Google Workspace. Organizations pay a subscription-based fee, typically per user, and avoid expenses related to server hardware, licensing, patching, backups, redundancy, and disaster recovery. This makes SaaS far more economical than deploying and maintaining email servers internally or in lower-level cloud models.

IaaS requires the organization to manage virtual machines, operating systems, email server software, and security configurations, which increases both cost and complexity. PaaS simplifies application development but is not designed for hosting ready-made services like email platforms. A VPC (Virtual Private Cloud) is a networking construct, not a service model, and does not inherently provide email hosting functionality.

The Network+ objectives emphasize selecting cloud service models based on cost, management responsibility, and operational efficiency. For standardized services like email, SaaS offers the lowest total cost of ownership and the least administrative burden, making it the best and most cost-effective choice.