Explanation
Choice D is the best answer. The convention being tested is colon use within a sentence. A colon used in this way introduces information that illustrates or explains information that has come before it. In this case, the colon gives additional information regarding the intriguing insight discovered by researchers, which is that the yellowfin tuna has a greater dietary diversity than the great white shark.
Choice A: It results in a run-on sentence. Two independent clauses (“Researchers … insight” and “in the realm … territory”) are fused without punctuation and/or a conjunction.
Choice B: It results in a run-on sentence. Two independent clauses (“Researchers … insight” and “in the realm … territory”) are joined by only a conjunction.
Choice C: It results in a comma splice. A comma cannot be used in this way to join two long independent clauses (“Researchers … insight” and “in the realm … territory”) such as these.
✨ Expert's Tip ✨
Remember the two functions of a colon in a sentence: 1) listing items and 2) giving additional information and explanation.