As written here (http://www.tizenhelp.com/download-whatsapp-messenger-app-on-samsung-z2/):
Now WhatsApp Messenger is available in Tizen Store for free of cost. On the Samsung Z2 you don’t have to download it, it already pre-installed on Samsung Z2. You can get it from Essential Apps folder on your device. It is one of the best social sharing app and very secure to use on Tizen, so you don’t have to worry about your privacy.