How to answer a call when my iPhone 5S is locked?

3rd Party developers are responsible for implementing the lock answer feature for their Apps. It appears Whatsapp added that feature a while ago:

http://www.idownloadblog.com/2016/09/14/wahtsapp-ios-10-update-siri-phone-suppor t/

Answer WhatsApp calls right from your lock screen, just like a standard phone call (iOS 10+)

http://www.iclarified.com/56874/you-can-now-answer-whatsapp-calls-right-from-you r-lock-screen

You may want to contact Whatsapp directly for support with their App and its features:

Contact WhatsApp

When locked and a call comes in, the screen should turn on and you should see a slider at the bottom. slide it to answer the call directly.

The call info will appear on the screen, you slide to answer if the device is locked.

You can also see this page from the iPhone User Guide. http://help.apple.com/iphone/10/#/iph3c9947bf

That would be an app issue with WhatsApp. I suggest you review their support for information on using their product.