Explanation
Correct answer: B.
Choice B is the best answer. “Previously” logically signals that the fused fiber theory came before Ramón y Cajal’s discovery.
Why the other choices are wrong
Choice A
Choice A is incorrect. “However” illogically signals that the fused fiber theory in this sentence contrasts with the information in the previous sentence. While this theory does contrast with Ramón y Cajal’s discovery, the previous sentence concludes by stating that his discovery went against prior assumptions about the brain. The fact that the fused fiber theory was one of those earlier assumptions makes “however” an illogical choice.
Choice C
Choice C is incorrect because “as a result” illogically signals that the fused fiber theory in this sentence was a result of the discovery in the previous sentence. Instead, the fused fiber theory came before Ramón y Cajal’s discovery.
Choice D
Choice D is incorrect because “likewise” illogically signals that the fused fiber theory in this sentence was similar to the discovery in the previous sentence. Instead, the fused fiber theory, which came before Ramón y Cajal’s discovery, was very different from it.