settings > data usage > data saver

[Standby]

In data saver there it is written that background apps wont run but when this option is on my WhatsApp is sending notifications that means WhatsApp is running in the background 

I haven't give the permission to WhatsApp to run in the background when data saver is on then too it is running in the background whyy ?????

I have shared the screenshot look on it 

[Sidhabhi]

Bro, that's normal notifications, go to whatsapp app info and check background data usage..

This option will cut down unnecessary usage of data, won't put apps to deep sleep mode..

[Ehsank]

you need to stop from WhatsApp separately by going into aop info and restrict background data and activity
but by enabling that you will not get any real time messages

[Ehsank]

this system only prevent background when your phone on idle mode for a long time