Do you know if they are using WebForms, MVC 5, or Core MVC (aka MVC 6)? They are all ASP.NET options and what you learn kind of depends on the answer.
If you do know, or can find out, what they are using then focus on that.
However, if you don't know what they use then I would say the safest option is to focus on MVC 5 as it's probably the most common ASP.NET option in use right now. WebForms isn't as common these days, but a lot of companies still use it so there's a chance that's what they want. If you have time, it wouldn't hurt to at least look at the basics of WebForms as well to give you an idea of how it works, just in case, but MVC 5 is the safer bet.
WebForms is quite different from MVC 5 (and Core MVC), so MVC 5 skills won't really transfer to WebForms skills but they may still hire you on and have you learn WebForms on the job. It's not an uncommon practice for companies.
It's unlikely they use MVC Core exclusively because it's only recently been released, but even if they did it I would expect they'd still hire someone for the position with MVC 5 experience. By learning MVC 5 you'll get familiar with a lot of the same concepts that Core MVC uses, so while there will still be a lot of new things to learn, the transition shouldn't be too bad.
Also, take some time to go over ASP.NET WebAPI as that's also commonly used.
In terms of learning resources, I would recommend Pluralsight, a video course website, as a place to start. It's $29 USD a month (or $299 USD a year) but you can get 3 months free through the Visual Studio Dev Essentials program. They have multiple courses on each ASP.NET option.