The sentence is correct as written. There are two issues evident from the original sentence. First, what follows the semi-colon must be a complete sentence in its own right. Choice (B) is wrong because the second half of the sentence is missing a verb that matches the subject "the British." Second, the question is testing your awareness of modifiers, as in "the British, who then relocated...." In the original sentence, this is correct.
Choice (C) is incorrect because without the word "then," the sentence loses the sense that the relocation of industry came as a result of the defeat. (D) is wrong because, while the word "who" follows the comma, there is nothing immediately before the comma (such as "the British") for the word "who" to refer to. Finally, (E) should be eliminated because "defeating" and "dismantled" are not parallel, while the structure of the sentence requires that they have the same form. Choice (A) is correct.