sat-dog

The correct answer is A. The sentence is a complete thought with a subject (the study), a verb (concluded), and an object (that exposure to natural environments can have a positive impact on mental health). This sentence introduces a new piece of information, so it needs a transition, and the transition should be a comma plus a coordinating conjunction. The conjunction "according to" is the best choice because it indicates that the new information is based on the previous information. The other choices are incorrect because they don’t follow the conventions of Standard English. Choices B, C, and D are incorrect because they use punctuation marks that are not used in Standard English to join two independent clauses in a sentence. Choice B is incorrect because the colon is used to introduce a list, a definition, or a series of items. Choice C is incorrect because the comma plus the comma cannot be used to join two complete sentences. Choice D is incorrect because a semicolon is used to connect two sentences or clauses that are related in meaning, but the clauses must be independent clauses. In this sentence, the first clause depends on the second for its meaning, so the semicolon is not the correct punctuation mark.