Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of punctuation within a sentence. The comma after “newts” pairs with the comma after “instance” to separate the supplementary element “for instance” from the rest of the sentence. The supplementary element appears between the prepositional phrase (“In…newts”) and the main clause of the sentence (“you…orientalis”) and signals that the sentence is presenting an example of how keys and values function in JSON files.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a sentence fragment (“In a JSON file storing data about fire belly newts”).
Choice C
Choice C is incorrect because a colon can’t be paired with a comma in this way to separate the supplementary element “for instance” from the rest of the sentence.
Choice D
Choice D is incorrect because a semicolon can’t be paired with a comma in this way to separate the supplementary element “for instance” from the rest of the sentence.