Choice D 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 group…plant”) and the supplementary element (“TN57…pests”) and to introduce the following elaboration on the specific varieties of sweet potato plants that were examined.

Choice A is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “TN57.”  Choice B is incorrect because a semicolon can’t be used in this way to join the main clause (“A group…plant”) and the supplementary element (“TN57…pests”). 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. Choice C is incorrect because it fails to mark the boundary between the main clause (“A group...plant”) and the supplementary element (“TN57...pests”) with appropriate punctuation.