Go to Settings > Mobile Data.

This brings you to the Mobile Data Section where you can select which apps will use Mobile Data and which app will not. This lets you manage mobile data flow and which app can consume data and which app cannot.

Videos sent over whatsapp which you may not want to watch also gets downloaded and consumes Mobile Data.

To prevent mobile data consumption ,

Open Whatsapp.

Select Settings on the bottom right corner.

Select Data and Storage Usage

Select Audio/Video/Photos and select appropriate options.

Wifi and Cellular would download the content over Wifi and Mobile Data

Wifi will download the content only on Wifi.

Never will never download the content unless you select it .