Explanation
Correct answer: B.
Choice B is the best answer. "In comparison" logically signals that the description of savannas in this sentence—that they are drier and their trees are spaced further apart—forms a comparison with the description of tropical forests in the previous sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect because "for instance" illogically signals that the description of savannas in this sentence exemplifies the description of tropical forests in the previous sentence. Instead, this description forms a comparison with the description of tropical forests.
Choice C
Choice C is incorrect because "firstly" illogically signals that the description of savannas in this sentence indicates the first in a series of things. Instead, this description forms a comparison with the description of tropical forests in the previous sentence.
Choice D
Choice D is incorrect because "in conclusion" illogically signals that the description of savannas in this sentence concludes or summarizes information in the previous sentences. Instead, this description forms a comparison with the description of tropical forests in the previous sentence.