None of the above. I don't create a separate notebook for each project because Evernote limits you to something like 250 notebooks (unless something has changed recently that I'm not aware of). Instead, I have one notebook for all of my projects and each project is a note within that notebook.
I also have a separate notebook for each context (calls, errands, home, computer, etc.) where I store all of the appropriate next action items.
If I have a particular project that requires support material beyond what I can comfortably input in or attach to a single note, I'll create a notebook for that project. The vast majority of mine don't require it, though.
If you really feel a need to create a "link" between projects and actions, you can create a keyword or words that you input in the body of each action. You can then find those actions using Evernote's robust and easy-to-use search feature. You can also use tags. I rarely do either, though, as I've learned for the most part to do without such linkages. I wouldn't suggest trying to use these workarounds for all of your projects as doing so would likely get unwieldy.
As I mentioned in another thread of yours about Evernote, the David Allen Company offers a terrific guide for setting up Evernote for use with GTD, available here:
https://store.gettingthingsdone.com/setup-guides-s/107.htm. It's only $10 and I found it to be well worth the price.