WhatsApp does not have the auto-reply feature, so WhatsReply for WhatsApp performs this task.
WhatsReply sends a predefined message to each contact that writes to you in automatized fashion and does it automatically when you are busy.
Additional features:
Connection status is not updated in WhatsApp (You will not be online if you are not)
You can set the time interval that has to pass from the last message to reply again, as well as the text of the message
Enable auto-reply only for contacts or also for WhatsApp groups