Explanation
Correct answer: A.
Choice A is the best answer. “A team…tried something new” is an independent clause leading to an explanation of what the new thing was. A colon can only be used at the end of an independent clause, and typically introduces further explanation that expands upon the first clause, which makes a colon the perfect choice here.
Why the other choices are wrong
Choice B
Choice B is incorrect. This choice results in a punctuation error. “Sequencing…dental plaque” can’t stand on its own as a sentence, and so it can’t be linked to the independent clause “a team…tried something new” with a semicolon. Only two independent clauses can be connected in this way.
Choice C
Choice C is incorrect. This choice results in a punctuation error. If “sequencing” is included in the first clause, it can no longer stand on its own as a complete idea. Since a colon can only come at the end of an independent clause, using one in this way creates an error.
Choice D
Choice D is incorrect. This choice results in a sentence fragment. “Sequencing…dental plaque” can’t stand on its own as a sentence.