AnjaliOberoi wrote:The computer company's present troubles are a result of technological stagnation,
marketing missteps, and managerial blunders so that several attempts to revise
corporate strategies have failed to correct it.

A. so that several attempts to revise corporate strategies have failed to correct it - 'so' is not related to anything here and it's unnecessary, also 'it' refers to troubles, so should be plural to refer correctly
B. so that several attempts at revising corporate strategies have failed to correct - same unrelated 'so' at the beginning
C. in that several attempts at revising corporate strategies have failed to correct them - 'in' sound really redundant here and it is not clear what 'them' is referring to 'corporate strategies', or 'troubles'
D. that several attempts to revise corporate strategies have failed to correct - right answer.
E. that several attempts at revising corporate strategies have failed to correct them- same mistake as at answer C

Answer D is indeed complete sentence, just try it different way - The computer company's present troubles are a result of actions that several attempts to revise corporate strategies have failed to correct.