Answer: D -Therefore
Step-by-step explanation:
We will plug in "Therefore" for "In sum" and see if it flows well and makes sense.
In sum, the union argued that Pullman had turned its workers into wage slaves.
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Therefore, the union argued that Pullman had turned its workers into wage slaves.
You can test it out yourself. When we "plug in" the other options in, D - Therefore makes the most sense.