What is a major challenge when posting forms with Sitecore MVC?

The correct choice highlights a significant complication that arises in Sitecore MVC when it comes to form handling within a single page request. In Sitecore MVC, each controller is designed to handle specific actions and often, in a complex application, multiple controllers may be required to process different parts of the same page. When forms are posted, and multiple controllers are invoked, this can lead to issues with maintaining the context of each controller's actions and can complicate the routing of requests.

This challenge can create a scenario where each controller may have its own set of actions and model states, making it difficult to manage the data flow, validation, and responses effectively. As a result, developers may face increased complexity as they try to coordinate these interactions and ensure that the correct controller handles the right part of the form submission.

While handling large data submissions, ensuring security, and managing session state are certainly important considerations in form handling, the specific challenge of managing multiple controllers within the same request context is often more intricate in terms of architecture and requires careful planning on how to structure controller logic and data flow effectively.