If your phone’s mobile plan only allow you to use a limited amount of mobile data, one of the way you will want to do is to make use of WiFi as much as possible to minimize the data usage. However, some of the apps can run in the background and use your mobile data without your knowledge. In this case, you might want to restrict that particular app from using your mobile data.
To do so, you have to first check out which app is using the most mobile data. If your phone is running Android 4.0 and above, you can go to “Settings -> Data usage” to view which app is using the most mobile data.
Once you have identified the app, tap on it and scroll all the way down to the bottom. Check the box “Restrict background data”. This will prevent the app from accessing your mobile data (it will still work in WiFi).