As an additional security measure on iOS 9 and newer, you can enable Touch ID or Face ID on WhatsApp. When enabled, you must use Touch ID or Face ID to unlock WhatsApp. You can still reply to messages from notifications and answer calls if WhatsApp is locked.
In order to use Touch ID or Face ID for WhatsApp, you must first enable it in iPhone Settings.
Note: If Touch ID or Face ID doesn’t unlock WhatsApp, you can enter your iPhone passcode.