The correct option is A either
Here, 'either' is correct because it is used to imply one or the other of two; it does not matter which. Here, it is one or other of the two - Spain or Greece.
'Both' is incorrect because it is used with plural nouns to mean ‘the two’ or ‘the one as well as the other’.
Neither' is incorrect because it implies not one nor the other of two things or people.
'Between' is incorrect because it is used to show a connection or relationship.