Android N will have native quick reply feature (called [*Direct Reply*](https://developer.android.com/preview/api-overview.html#notification_enhancements)), and while WhatsApp will probably use it, it won't work for pre-N (although it has implemented [its own quick reply from notification for all Android version](http://android.stackexchange.com/questions/61577/show-whatsapp-popup-when-clicking-the-message-in-notification-panel?lq=1)).

However, there are some apps that can emulate the direct reply feature on Android N:

- [quickReply](https://play.google.com/store/apps/details?id=com.jamworks.quickreply) (free, 5.0+)

 > quickReply will show you separate notifications for every contact including reply action buttons, so you can easily write quick replies without opening the messaging app.

 > **QUICKREPLY SUPPORTED APPS**:  
 > WhatsApp, Telegram, Threema, Skype, Line, WeChat, Kik, EvolveSms, YAATA, Textra, ChompSms, Hangouts, Handcent SMS... 

- [Floatify - Quick Replies](https://play.google.com/store/apps/details?id=com.jamworks.floatify) (free, 4.3+)

 > Advanced heads-Up notifications for your phone! QuickReply lets you send messages directly from the notification! ...

 > **MAIN FEATURES**:  
 > Heads-up Quick Replies (5.0+, limited on 4.3+, see below for supported apps)

 > **QUICKREPLY REPORTED WORKING**:  
 > WhatsApp, Telegram, Threema, Skype, Line, WeChat, Kik, EvolveSms, YAATA, Textra, ChompSms, Hangouts, Handcent SMS... 

<sup>Both apps are developed by Jawomo, so it's not really surprising they have similar feature and supported apps...</sup>