Here's the official explanation provided by the GMAC for this question:

This sentence describes the comet Crommelin and Halley's comet each passing Earth at about the same distance and in about the same position as each other. Idiomatically, in this context the adverbial comparative construction starting with at about the same … should be completed with as, not with that. This construction must be clearly used to compare how the comets pass, rather than to compare a comet itself with a distance or position.

Option A: Idiomatically, the adverbial construction starting with at about the same … should be completed with as, not with that. The construction the same … that results in a comparison whose implicit terms are two adverbial constructions (at the distance and in the position the comet Crommelin will pass) and two noun phrases (the distance and the position Halley's comet will pass). This comparison's terms are not grammatically parallel.

Option B: Idiomatically, the adverbial construction starting with at about the same … should be completed with as, not with that. The verb form is to be passing is unnecessarily convoluted, and might be read as suggesting that somebody has mandated that the comet should pass.

Option C: This seems to compare the distance at which and the position in which the comet Crommelin passed with Halley's comet itself, rather than with the distance at which and the position in which Halley's comet will pass.

Option D: Correct. In context, as will Halley's comet is an appropriate abbreviation for as the distance at which and the position in which Halley's comet will pass, correctly completing the comparison with the distance at which and the position in which the comet Crommelin passed.

Option E: The word do is unnecessary and inappropriate in this context. It confusingly suggests that Halley's comet will "do" about the same distance at which and about the same position in which the comet Crommelin passed.

The correct answer is D.

Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.
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