Q: I m using iphone 5s and how can i lock individual app on ios 9.3.1

Please explain what you mean by "lock" an app. What is it that you're trying to accomplish?

Only apps that are programmed to do that.

You cannot locked stock apps that came with the iPhone.

Except for the Notes app.

Like the others who responded before me, I am puzzled by what you mean by "lock individual app".

There is some chance you might mean restrict an iPhone user to one individual app, in which case, here is how to do that.

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support

deggie wrote:

 

Except for the Notes app.

Not the app itself though just the individual notes.

Which is what the OP may be trying to do.

Maybe, we'll see.

You would need to contact the developers on whatsapp, Facebook, etc. and ask them to add this feature.