Question from the Magento test

Both of them will be able to complete checkout successfully, in case the second customer will add product to the cart before first completes checkout. Product quantity in stock will be set to -1 afterwards.

If both customers add this product to cart, none of them will be able to complete checkout.

Only first customer will be able to complete checkout. Product quantity in stock will be set to 0 afterwards.

Magento’s behaviour for this situation can be configured in settings.

This situation is impossible, because after last product item was added to the first customer's cart, product becomes unavailable for purchase any more.