How make a asp-for dropdown list

I'm new to MVC core. This is my first time using it. I'm instructed to use asp-for tags. The closes I can find is:

<select asp-for="Activity" style="width:90%"><option value="CA">Canada</option><option value="US">US</option><option value="MEX">Mexico</option><option value="ENG">England</option></select>

But I need to have the list dropdown, how do you do that? What are the steps?