How can I uncheck or reset the radio button?

if the Radiobutton belong to a radiogroup you can NEVER unchecked alone that button programaticaly, so you must to do a method to unched all first.

    public void unchecked()
     {
     RadioGroup x=findViewById(R.id.NameOfRadiogroup);
     x.clearCheck();
     }  

and then you must to call the method.

I think the problem is with btn_practicerg object i.e if you are creating new RadioGroup object everytime you display a new question then you have to execute RadioGroup.clearCheck() on new btn_practicerg object instead of old one that I think you are doing currently.

Even better:

    btn_practice1.setText(answ1.get(0));
    btn_practice2.setText(answ1.get(1));
    btn_practice3.setText(answ1.get(2));
    btn_practice4.setText(answ1.get(3));
    btn_practice1.setChecked(false);
    btn_practice2.setChecked(false);
    btn_practice3.setChecked(false);
    btn_practice4.setChecked(false);

to uncheck all the buttons in the beginning. I hope this will solve your problem.

Put all your buttons in a RadioGroup then when you need to clear them all use RadioGroup.clearCheck();