In Facebook Messenger, when you are Chatting with one person, there is an option of Secret Chat. As you know, WhatsApp is the Child company of Facebook and now this feature will come to WhatsApp but only for Groups for now.
First, WhatsApp gives Slide to Reply option and later WhatsApp Stickers. But now, WhatsApp goes to launch Privately Reply for groups members, during the chatting with a person on WhatsApp group.
How to enable?
Just Tap & hold on a message and after selecting the message. Click on upper right side three dots buttons and click on Reply Privately.
This feature is only available for Beta Users for now, but it will go public for normal users soon.