Let's Start -:
1) Firstly Take Your Android phone
2) Go To "Setting"
3) Then Select "Data Usage"
4) Press Option And Tick on "Restrict Background Data"
5) That's It! Problem Solved, Now Your Phone Will Take Only Single app Data
(Means - When You Enable Background Restrict Then no background app Will Take Any Data, Only 1 App Will Take Your Data Which Are running)