No app can hack WhatsApp, and every product advertising exactly that is selling either a keylogger that needs the phone unlocked in your hand, a repackaged Android build that needs the account's six-digit verification code, or nothing at all. WhatsApp messages are end-to-end encrypted, so there is no network-side path in, and a vendor promising remote WhatsApp reading is describing a capability that does not exist on any phone.
If the account is yours, forget "hacking" and use recovery. Reinstall WhatsApp and verify with the SMS or call code sent to your number; if the SIM is gone, get a replacement SIM from your carrier first, because the number is the account. On Android, Google's Find My Device can unlock or reset the handset; on an iPhone, Apple Account recovery and Find My handle the same job. Then open WhatsApp and check Linked devices - anything you do not recognise there has been reading your chats in real time, and removing it is one tap. Official help is at whatsapp.com/contact.
If it is a phone you administer for a minor, the honest check is the phone itself: a parent holding the child's device reads the chat in front of them, which is disclosed oversight and needs no software. Beyond that, a WhatsApp backup stored in the child's Google Drive or iCloud can be restored to a spare handset, but only where you control the account - not by guessing someone's password.
If it is an adult's phone, you have a legal risk, not a software problem. Covert access to another adult's messages is caught by the US Wiretap Act (18 U.S.C. 2511) and state privacy statutes, the UK Computer Misuse Act 1990, GDPR in the EU, and stalking or domestic-abuse laws that treat device surveillance as an aggravating factor.
Where our own product fits: Ultimate Phone Spy does not break encryption either. On an Android 4.0+ phone, Windows PC or Mac you own or administer, it gives one dashboard for SMS, call logs, location, web history and installed apps (parental control features, system requirements). It has no iPhone support, because iOS gives no device-level access - and anyone claiming otherwise about WhatsApp has the same problem.