Answer: D) "So-called anonymous tracking is not very secure—the anonymity is fairly easily broken. Cracking [it]...is a fairly easy engineering feat."
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You want to omit words to shorten the sentence, but you also want to make sure you're keeping the general idea of the sentence the same!
For choice A, the only thing different is that there is an ellipsis, "..." instead of a dash. This choice doesn't omit any words, so A is wrong.
For choice B, you can see that words are omitted in the second sentence, but now the sentence doesn't make sense. "Cracking open that anonymous...is a fairly easy engineering feat." That anonymous what? So choice B is wrong.
For choice C, words are being omitted, but what the quote is trying to say is being changed. The quote is saying that anonymous tracking is not secure because people can be easily identified. Choice C, however, is saying that anonymous tracking is easy. That is not what the original quote was trying to say, so C is wrong.
That leaves choice D, which correctly omits words and summarizes what the original quote said!
