How to use a radiobutton with IF statement?

I´m trying use RadioButton with statement IF

in short: I created a WPF application in which I want to select one of the four options and then click the button to open another window.I'd like to make sure the user clicked one off.

private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (here I don't know how to create a condition)
            {
                MessageBox.Show("Musíš zvolit, jednu z početních operací!");
            }
            else
            {
                uroven lvl = new uroven();
                lvl.Show();
                this.Close();
            }
        }

RadioButton == unchecked I thought of this condition but it does not go as well as the possibility of or(||)

as seen from the code I use error message in if and else opens another window

I'll be happy for any advice

Thank you