Respuesta :
"They were from different geographic and cultural parts of Spain" is the one reason among the following choices given in the question why the marriage of Ferdinand and Isabella helped unify Spain. The correct option among all the options that are given in the question is the last option or the fourth option or option "D".
The marriage of Ferdinad and Isabella helped unify Spain because they were from different geographic and cultural parts of spain. Therefore, D would be the correct answer choice.