.NET Core is a cross-platform, high-performance framework for building modern, cloud-based, and internet-connected applications. Key features include its open-source nature, modular architecture, cross-platform support, high performance, and the ability to deploy applications to different operating systems.

State management in ASP.NET can be handled through various techniques such as ViewState, SessionState, ApplicationState, and caching. Choosing the right method depends on the scope and lifespan of the data being managed, as well as performance considerations.

ASP.NET MVC is a framework for building web applications using the Model-View-Controller pattern, promoting separation of concerns and testability. It provides more control over HTML and better support for agile development. ASP.NET WebForms, on the other hand, abstracts the HTML, offering a more event-driven and RAD (Rapid Application Development) approach, but with less control over the HTML and less support for testability.

Ensuring security in a .NET application involves implementing various measures such as input validation, authentication and authorization, encryption, secure communication protocols (HTTPS), and regular security updates. Using frameworks like ASP.NET Identity and libraries such as OWASP can further enhance security.

A detailed answer showcasing the candidate’s problem-solving skills, ability to handle stress, and technical expertise. Look for specifics on the challenges faced, the strategies employed to overcome them, and the successful outcomes achieved