Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of punctuation within a sentence. The comma after “observation” pairs with the comma after “however” to separate the supplementary adverb “however” from the rest of the sentence, which consists of a main clause (“Lai...observation”) followed by a participial phrase (“recommending...study”).
Why the other choices are wrong
Choice A
Choice A is incorrect because a semicolon can’t be used in this way to join the main clause (“Lai...observation”) and the participial phrase (“recommending...study”) that follows the supplementary adverb “however.” A semicolon is conventionally used to join two main clauses, and “recommending...study” isn’t a main clause.
Choice C
Choice C is incorrect because placing a period after “observation” results in a rhetorically unacceptable sentence fragment beginning with “however.”
Choice D
Choice D is incorrect because a semicolon can’t be used in this way to join the main clause (“Lai...however”) and the participial phrase (“recommending...study”) that follows. A semicolon is conventionally used to join two main clauses, and “recommending...study” isn’t a main clause.