<p>As early as 1865, there had been proposals for a New York subway, but (that) took decades to resolve the many political, financial , and technical problems. A . No change b. it. C. Those. D. Delete The answer is b. I chose c because I just thought proposals are plural, and it might fit the sentence better. Thanks for helping !</p>
<p>The question is what does (that) refer to. It does not actually refer to the proposals. Here’s another way to write the sentence:</p>
<p>As early as 1865, there had been proposals for a New York subway, but (history) took decades to resolve the many political, financial, and technical problems.</p>