Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used correctly to mark the boundary between one sentence (“The Balkan...three”) and another (“Known...chairperson”). While the supplementary element (“known...Croats”) could logically be attached to either the first main clause (“The Balkan...three”) or the second main clause (“these elected...chairperson”), end-of-sentence punctuation is needed to indicate to which sentence the supplementary element belongs.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice. In this choice, the first sentence, consisting of a main clause (“The Balkan...three”) and supplementary element (“known...Croats”), is incorrectly joined with a second main clause (“these elected...chairperson”) using a comma. A comma can’t be used in this way to join two sentences.
Choice C
Choice C is incorrect because it results in a comma splice. In this choice, the first sentence, consisting of a main clause (“The Balkan...three”) and a relative clause (“who are...Croats”), is incorrectly joined with a second main clause (“these elected...chairperson”) using a comma. A comma can’t be used in this way to join two sentences. Additionally, this choice omits the comma needed after “three” to mark the beginning of the supplementary relative clause (“who are known...Croats”).
Choice D
Choice D is incorrect because it results in a comma splice. In this choice, the first sentence, consisting of a main clause (“The Balkan...three”) and a supplementary relative clause (“who are...Croats”), is incorrectly joined with a second main clause (“these elected...chairperson”) using a comma. A comma can’t be used in this way to join two sentences.