Can Google analytics track which specific post people opted in?

I think that optin form plugin should have that feature

Yes it can. Set goals and tags to track that.

You can create event using Google Tag Manager.

Nice Information!!
Can anyone tell me How to check my exact organic traffic from (not provided) section?
Thanks in advance.

Yes it can be done easily via tag manager, you just need to learn more about it.

To further elaborate on what others have said, Google Tag Manager can allow you to track this.
You can set up a tag events to trigger when someone clicks on a specific button (or submits a form).

I have done this in the past to track how many people simply click register vs how many people register and click the verification link we sent them (finish setting up their account). To do so you need to set up a tag even with a "Click" Action. Then you would set up a Trigger to fire once the button is clicked. You can specify a url here, so you would only see that event when a button is clicked from that specific blog post/page.

Someone else may have a better way to do this (I have only used this with one button), but I think if you are trying to see this from all your different posts, you may need to set up an individual tag and trigger for each button that you are looking for it to fire on. -- Maybe there is a way for Google Analytics to track which URL this event triggers from?

Hopefully this gets you moving a little better in the right direction.

- Nick