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.
 \/
 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.