C. where no considerable European settlement is possible
Though all the passage clearly expresses the author's idea to convey that British is superior to all countries, option C is the phrase that most clearly suggest that.
By using the words "no (...) is possible", the author expresses that normally people considered impossible/unfeasible a big European settlement in their lands, but since they are the British Empire, they are superiors and therefore they are the ones who can make it possible.