Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is punctuation use between a main clause and a supplementary phrase. In this choice, a colon is correctly used to mark the boundary between the main clause (“A study…continents”) and the supplementary phrase (“geological…above”) and to introduce the following explanation of the origin of Earth’s continents.
Why the other choices are wrong
Choice A
Choice A is incorrect because it fails to mark the boundary between the main clause (“A study…continents”) and the supplementary phrase (“geological…above”) with appropriate punctuation.
Choice C
Choice C is incorrect because a semicolon can’t be used in this way to join the main clause (“A study…continents”) and the supplementary phrase (“geological…above”). A semicolon is conventionally used to join two main clauses, whereas a colon is conventionally used to introduce an element that explains or amplifies the information in the preceding clause, making it the better choice in this context.
Choice D
Choice D is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “geological.”