Open MS word in web application

You can't.
Any application you open via C# will be running on the Server, because that is where your C# code executes. As a result - just like MessageBox - it's not visible to the CLient at all. (It may appear to work in dev, because the Client and the Server are both executing on the same machine, but it will fail completely in production). In addition, most web servers do not have any work of "user software" installed and that includes Office.

You cannot open any application on the client directly from the server (or from the browser) for security reasons. Think about it: if your site could open an app on your computer, so could malicious sites - and then the ransomware problem would be a whole load worse than it is.