Tap "Download." This will install the last version of WhatsApp that was compatible with iOS 7.1.2.
Older operating systems like iOS 7 lack the necessary security patches to support modern end-to-end encryption.
You will see websites offering free downloads of modified WhatsApp versions (like WhatsApp Delta or WhatsApp++). They are not on the App Store for a reason:
I can provide a step-by-step guide for the specific route you choose.