Explanation
Correct answer: D.
Choice D is the best answer. "As such" correctly signals that the claim in this sentence—that Ostrom’s work is a repudiation of the "tragedy of the commons" view—follows logically from the information about Ostrom’s studies in the previous sentence. According to that sentence, Ostrom’s studies demonstrate that common pool resources can in fact be sustainably managed by the people who use them.
Why the other choices are wrong
Choice A
Choice A is incorrect because "by contrast" illogically signals that the information in this sentence contrasts with the information about Ostrom’s studies in the previous sentence. Instead, the claim that Ostrom’s work repudiates the "tragedy of the commons" view follows logically from that information.
Choice B
Choice B is incorrect because "for example" illogically signals that the claim in this sentence exemplifies the information about Ostrom’s studies in the previous sentence. Instead, the claim that Ostrom’s work repudiates the "tragedy of the commons" view follows logically from that information.
Choice C
Choice C is incorrect because "that said" illogically signals that the information in this sentence is an exception or caveat to the information about Ostrom’s studies in the previous sentence. Instead, the claim that Ostrom’s work repudiates the "tragedy of the commons" view follows logically from that information.