The answer is B: They had better military technology.
For the most part, Western countries were able to colonize other countries because they had superior military technology which made resistance on the side of the colonized futile. One exemplary case, which set the scene for colonization in great scale all around the world, is the case of the Spanish Crown conquering Mesoamerica, where Europeans had already mastered the use of gun powder and horsemanship and the natives of Mesoamerica had not, making the latter extremely vulnerable to the invasions of the former.