To solve this question, the user needs to know the different types of cloud computing services. The three main types of cloud computing services are Software as a Service (SaaS), Infrastructure as a Service (IaaS), and Platform as a Service (PaaS). SaaS offers software applications over the Internet, IaaS offers virtualized computing resources over the Internet, and PaaS provides a platform for developers to build, test, and deploy applications over the Internet.

Now, let's go through each option and explain why it is right or wrong:

A. Force.com: This option is a PaaS service offered by Salesforce.com. Force.com provides a platform for developers to build and deploy web applications and mobile apps over the Internet. Hence, this is a Platform as a Service.

B. Microsoft Azure: This option is a cloud computing platform and Infrastructure as a Service (IaaS) service offered by Microsoft. Azure provides virtualized computing resources over the Internet, such as virtual machines (VMs) and storage. Thus, this is an Infrastructure as a Service.

C. Amazon EC2: This option is also an IaaS service offered by Amazon Web Services (AWS). Amazon EC2 provides virtualized computing resources over the Internet, such as VMs and storage. Thus, this is an Infrastructure as a Service.

D. Joyent: This option is a cloud computing company that offers Infrastructure as a Service (IaaS) and Platform as a Service (PaaS) services. Joyent provides virtualized computing resources over the Internet, as well as a platform for developers to build and deploy applications over the Internet. Hence, this is both Infrastructure as a Service and Platform as a Service.

Therefore, the answer is: C. Amazon EC2, since it is not a Platform as a Service, but an Infrastructure as a Service.