If you phone is an Apple iPhone, those are pretty secure.
If you want to Reset you iPhone, here is the path.
Settings>General>Transfer or Reset iPhone>Erase All Content and Settings
This process will restore the phone to the condition it has when it left the factory with the possible exception a newer version of iOS will be installed.
If you trust WhatsApp, that will be part of the system when you Restore. The WhatsApp software will be a fresh download, but any data from the WhatApp system will be whatever they have in their system.
If you do not trust WhatsApp, you should delete WhatsApp from your iPhone and make another backup before you Reset your phone. Then your Restore will not have WhatsApp.