Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of verbs to express tense in a sentence. In this choice, the past tense verb "was," used in conjunction with the prepositional phrase "from outside the United States," correctly indicates that in the 1860s nearly 30% of the population in Los Angeles wasn’t from the US.
Why the other choices are wrong
Choice A
Choice A is incorrect because the present progressive tense verb "is being" doesn’t indicate that the claim about Los Angeles’s population refers to the population back in the 1860s.
Choice C
Choice C is incorrect because the present tense verb "is" doesn’t indicate that the claim about Los Angeles’s population refers to the population back in the 1860s.
Choice D
Choice D is incorrect because the future tense verb "will be" doesn’t indicate that the claim about Los Angeles’s population refers to the population back in the 1860s.