Eli Eisenberg, a genetics expert at Tel Aviv University in Israel, recently discovered that ______ blank

Choice D is the best answer. The convention being tested is the punctuation of a supplementary element within a sentence. In this choice, the dash after "cephalopods" pairs with the dash after "cuttlefish" to clearly separate the supplementary element "ocean dwellers that include the squid, the octopus, and the cuttlefish" from the rest of the sentence. This supplementary element functions to explain what cephalopods are, and the pair of dashes indies that this element could be removed without affecting the grammatical coherence of the sentence.

Choice A is incorrect because it fails to use appropriate punctuation to separate the supplementary element that explains what cephalopods are from the rest of the sentence. Choice B is incorrect because it fails to use appropriate punctuation to separate the supplementary element that explains what cephalopods are from the rest of the sentence. Choice C is incorrect because it fails to use appropriate punctuation to separate the supplementary element that explains what cephalopods are from the rest of the sentence.