OFFICIAL SOLUTION

From your initial scan of the problem, this one should scream "Subject-Verb Agreement" at you with the first decision point being "pose" vs. "poses." What is the subject of that verb? It's "cycle" ("of eviction and relocation of immigrant families" just tells you more about the cycle). That means that you need the singular verb "poses," and that limits you to A vs. D.

Between A and D, choice D uses the modifier "the tool" to describe "education" - a logical modifier and clear meaning to the sentence. In chioce A, "and the tool" is at best ambiguous (does "and" connect "obstacle" and "tool" as two things that the cycle poses? Or "education" and "the tool" as two separate things that are facing obstacles?) and at worst illogical (the logical meaning is that education is the tool, not that education and this vague "tool that is most likely..." are two separate things). Therefore, answer choice D is correct.
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You're close in thinking that it's a comma splice!

But take another look at the reconstructed sentence: "There is little doubt that the ongoing cycle of eviction and relocation of immigrant families poses a significant obstacle to education, the tool most likely to enable those families to break that cycle." Remember that in order for a clause to be a complete sentence, it needs to have a subject and a main verb. Because "the tool most likely to enable those families to break that cycle" doesn't have a main verb (it is a noun with a descriptive phrase after it), it can't be a complete sentence.

So instead of a comma splice, what you have here is a modifier. "The tool most likely to enable those families to break that cycle" modifies the word "education," the word before the comma.
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