The current version of WhatsApp can be downloaded from the App Store. This is the only version available for download, and contains the newest features and bug fixes.
If you have any problems with your Apple ID, password, or downloading in general, please contact Apple Support. Apple controls the download and installation process for all iPhone apps, and we do not have access from our end to resolve any App Store related issues. Here's a link to their support page: http://www.apple.com/support/ios/contact/
For more information on what devices and iOS versions are supported, please read this article.
If you receive a message about Family Sharing when trying to download WhatsApp, this is an issue with the new "Family Sharing" feature in the App Store. We have no way to resolve this from our end. Please contact Apple Support for assistance.