score:4

Accepted answer
var filtered = MobileList.Select(m => new MobileModel {
                Model = new ObservableCollection(m.Model.Where(model => model.Year == "2011").ToList()),
                Brand = m.Brand,
                OS = m.OS
            })

;

score:2

I think you can use inner where conditions for this.Pls check this:

   var result = MobileList.Where(m => m.Model.Where(mdl => mdl.Year =="2011")).ToList();

score:6

use SelectMany() :

MobileList.SelectMany(x => x.Model).Where(m=>m.Year==2011)


Related Query

More Query from same tag