Pre-conditions for Scrum Planning to begin - Question

Hello All!

There is a question in Product Owner Open Assessment about what pre-conditions must be fulfilled to allow Scrum Planning event to begin.

And the possible answers include:

1) Fully refined Product Backlog

(which is obviously incorrect)

2) Formal budget approval

(also incorrect)

3) Clear, non-negotiable Sprint goal

(also very incorrect for very obvious reasons)

4) Enough "Ready" Product Backlog to fill the Sprint

(which is the incorrect answer, but I really don't understand why??)

5) There are no pre-conditions.

So, the correct answer is (5), but, according to a Scrum Guide on Scrum Planning:

"The input to this meeting is the Product Backlog"

Then, from guide "After the Development Team forecasts the Product Backlog items it will deliver in the Sprint, the Scrum Team crafts a Sprint Goal." - how Dev Team can select items from backlog which is not READY enough? :)

So, logically thinking, the Product Backlog, HAS to be READY enough to use it as an input for Scrum Planning....

Why then the option (4) is incorrect?

thank you!!