Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the punctuation of elements in a complex series. It’s conventional to use a semicolon to separate items in a complex series with internal punctuation, and in this choice, the semicolon after “significance” is conventionally used to separate the first item (“intricate...significance”) and the second item (“depictions...animals”) in the series of common elements found in Celtic art. Moreover, the semicolon after “significance” matches the semicolon used later to separate the second item (“depictions...animals”) and the third item (“and...accents”) in the series.
Why the other choices are wrong
Choice B
Choice B is incorrect because a comma after “significance” doesn’t match the semicolon used later to separate the second and third items in the series (“depictions...animals” and “and...accents”).
Choice C
Choice C is incorrect because a colon can’t be used in this way to separate items in a complex series.
Choice D
Choice D is incorrect because placing a period after “significance” results in a rhetorically unacceptable sentence fragment beginning with “depictions.”