Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is end-of-sentence punctuation. This choice correctly uses a period to punctuate a declarative sentence that asks an indirect question ("The toxic nature of some of these materials recently led a team from the University of Sheffield to investigate how their use could be better regulated").
Why the other choices are wrong
Choice A
Choice A is incorrect because the structure requires a period and a declarative clause at the end of the sentence that states what the team investigated, not an interrogative clause that asks a direct question, such as "how could their use be better regulated?"
Choice C
Choice C is incorrect because it’s unconventional to use a question mark in this way to punctuate a declarative sentence.
Choice D
Choice D is incorrect because the structure requires a declarative clause at the end of the sentence that states what the team investigated, not an interrogative clause that asks a direct question, such as "how could their use be better regulated?"