Why is cooperative binding of hemoglobin adaptive?

For example, when an oxygen atom binds to one of hemoglobin's four binding sites, the affinity to oxygen of the three remaining available binding sites increases; i.e. oxygen is more likely to bind to a hemoglobin bound to one oxygen than to an unbound hemoglobin.

Is there possibly mutliple choice answers or is this a type/write it in question?

The correct answer is d