I have windows 8.1 OS. and when I choose Windows Phone App in VS it create Windows Phone 8.1 App without choice
In the Windows Phone templates in Visual Studio, you'll find two types of templates: Windows Phone
and Windows Phone Silverlight
(see the image below).
The Windows Phone
templates refer to the XAML OS platform, which is only available for Windows Phone 8.1.
The Windows Phone Silverlight
templates refer to the Silverlight OS platform, which is available for both 8.0 and 8.1. So if you want to create Windows Phone 8.0 app, you'll need to use one of these templates.