BLMYou kinda have a case here. Sure, there are inverted structures like this on the GMAT, but usually there is only one valid way to resolve the issue. For instance, if the first part were just a modifier ("Bringing Twain out"), then we'd 100% need to have "were," since we'd clearly have a compound subject (HELP and A TOUR) after the verb. In C, however, we can choose to read the first part as the true subject, and then to see "help and a tour" as comprising what helped Twain. I still might prefer D, but a proper GMAT SC question should have 4 clearly wrong answers, and unfortunately this one word is the only difference between C and D. That in itself is also a problem; a real question should have multiple differences in an underline of this length, including at least one at the beginning and one at the end.
Additionally, all of the choices have a shared grammatical error. "After losing" is an adverbial modifier, and since it's Twain that lost his fortune, there should be an action by Twain for this modifier to apply to. Since there isn't, we're forced to read this part as saying that the things that got Twain out of trouble (help and a tour) were also what lost his fortune. This doesn't make sense!
I'd strongly advise everyone to spend this last month of SC on official questions. There are enough out there to satisfy anyone!