Almost there!

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Labo6Extra
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void checkBox1_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxSecond.IsChecked == true)
            {

                cbxFirst.IsChecked = false;
                cbxThird.IsChecked = false;
                cbxFourth.IsChecked = false;
            }
        }

        private void txbName_TextChanged(object sender, TextChangedEventArgs e)
        {

           
            if ((txbName.Text == "Tim") && (txbFamName.Text == "Dams") && (txbLocation.Text == "AP") && (txbPassword.Text == "C#") && (cbxThird.IsChecked == true) 
                && (cbxA.IsChecked == true) && (cbxApple.IsChecked == true))
                MessageBox.Show("Je hebt het slot kunnen kraken!");
        }

        private void cbxFirst_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxFirst.IsChecked == true)
            {

                cbxSecond.IsChecked = false;
                cbxThird.IsChecked = false;
                cbxFourth.IsChecked = false;

            }
        }

        private void cbxThird_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxThird.IsChecked == true)
            {
                cbxFirst.IsChecked = false;
                cbxSecond.IsChecked = false;
                cbxFourth.IsChecked = false;
            }
        }

        private void cbxFourth_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxFourth.IsChecked == true)
            {
                cbxFirst.IsChecked = false;
                cbxSecond.IsChecked = false;
                cbxThird.IsChecked = false;
            }
        }

        private void cbxA_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxA.IsChecked == true)
            {
                cbxB.IsChecked = false;
                cbxC.IsChecked = false;
                cbxD.IsChecked = false;
            }
        }

        private void cbxB_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxB.IsChecked == true)
            {
                cbxA.IsChecked = false;
                cbxC.IsChecked = false;
                cbxD.IsChecked = false;

            }
        }

        private void cbxC_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxC.IsChecked == true)
            {
                cbxA.IsChecked = false;
                cbxB.IsChecked = false;
                cbxD.IsChecked = false;
            }
        }

        private void cbxD_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxD.IsChecked == true)
            {
                cbxA.IsChecked = false;
                cbxB.IsChecked = false;
                cbxC.IsChecked = false;
            }
        }

        private void cbxApple_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxApple.IsChecked == true)
            {
                cbxBanana.IsChecked = false;
                cbxCitron.IsChecked = false;
                cbxOrange.IsChecked = false;

            }
        }

        private void cbxBanana_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxBanana.IsChecked == true)
            {
                cbxApple.IsChecked = false;
                cbxCitron.IsChecked = false;
                cbxOrange.IsChecked = false;

            }
        }

        private void cbxCitron_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxCitron.IsChecked == true)
            {
                cbxApple.IsChecked = false;
                cbxBanana.IsChecked = false;
                cbxOrange.IsChecked = false;
            }
        }

        private void cbxOrange_Checked(object sender, RoutedEventArgs e)
        {
            if (cbxOrange.IsChecked == true)
            {
                cbxApple.IsChecked = false;
                cbxBanana.IsChecked = false;
                cbxCitron.IsChecked = false;
            }
        }
    }
}