Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of verb forms within a sentence. The nonfinite to-infinitive verb “to study” is correctly used to form a subordinate clause that indicates what Dr. Caballero-Gill uses chemistry and fossil evidence for (to study oceans as they were in the past).
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in an ungrammatical sentence. The finite verb “has studied” can’t be used in this way to form a subordinate clause that indicates what Dr. Caballero-Gill uses the evidence for.
Choice C
Choice C is incorrect because it results in an ungrammatical sentence. The finite verb “studied” can’t be used in this way to form a subordinate clause that indicates what Dr. Caballero-Gill uses the evidence for.
Choice D
Choice D is incorrect because it results in an ungrammatical sentence. The finite verb “studies” can’t be used in this way to form a subordinate clause that indicates what Dr. Caballero-Gill uses the evidence for.