Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of punctuation within a sentence. This choice uses a semicolon in a conventional way to join the first main clause (“That...Chulalongkorn”) and the second main clause (“deterring...achievement”).
Why the other choices are wrong
Choice B
Choice B is incorrect. Using the conjunction “that” to introduce “deterring...achievement” as a subordinate element of the main clause (“That...Chulalongkorn”) results in a confusing and ungrammatical sentence.
Choice C
Choice C is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses.
Choice D
Choice D is incorrect because it results in a run-on sentence. The two main clauses are fused without punctuation and/or a conjunction.