While the inability to conduct security assessments against a service provider is indeed a risk when adopting a SaaS solution, it may not be the greatest risk in comparison to other risks. In many cases, SaaS providers have their own security assessment processes and are often required to comply with industry standards or certifications, which can help ensure a certain level of security.
However, the inability to obtain company data when migrating to another service (option C) can have more severe consequences, such as data loss, increased costs, and delays in business operations. This risk can directly impact the company's core business processes and data, making it a greater risk to consider when adopting a new SaaS solution.