prevent users from showing XML file on server .NET

If it's going to be available to the client via Javascript, it's got to be available to the client in general. Now you could encrypt it, then decrypt it in the Javascript - but that's more of an obfuscation technique than anything else... basically if you need the browser to have the plaintext version of the file at some point, you can't prevent it from being available to users in general.

If the problem is that the file contains both questions and answers - i.e. data that you're happy for users to see and also data that you don't want them to see - you should split the file in two.

If you only want users to be able to see the data at the right time (i.e. when the question is asked) then you can introduce a server-side aspect which will serve one question at a time, when requested, and log that the user has just seen the question. That won't stop users from fetching questions, but it will stop them from doing so in advance without you being aware of it. (Unless, of course, they can log in as a different user, fetch the questions, work out the answers, then log in as their real account and give all the right answers. At that point it's more a matter of authentication and user control than anything else - after all, they could take the test twice, too...)