Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of punctuation within a sentence. In this choice, the colon correctly introduces the series of goals held by the 1919 Inter-Allied Women’s Conference.
Why the other choices are wrong
Choice B
Choice B is incorrect because placing a period after "threefold" results in a rhetorically unacceptable sentence fragment beginning with "To."
Choice C
Choice C is incorrect because it results in a run-on sentence. The main clause ("The conference’s goals were threefold") and the series supplement ("to…rights") are fused without punctuation.
Choice D
Choice D is incorrect because a semicolon can’t be used in this way to introduce a series. A semicolon is conventionally used to join two main clauses, whereas a colon is conventionally used to introduce a series, making the colon the better choice in this context.