An analysis of the question asks valid queries to be answered.
The Original Sentence -Though viewed from a distance, Saturn's main rings may appear to be smooth and continuous, they are in fact composed of thousands of separate icy ringlets when viewed up close.
Though is normally used to show some contradiction or to show opposing views/opinions. E.g. Though not a great ground fielder, (but) Shane Warne was an excellent slip catcher.
In the original sentence the placement of though does not make the opening phrase look correct. As cited in the Warne example, the contradiction needed to be stated after the comma. Instead the phrase is awkwardly modifying the subject- Saturn's main rings.
The option B - Though Saturn's main rings may appear smooth and continuous when viewed from a distance, they are in fact composed of thousands of separate icy ringlets when viewed up close.
In this option Though is placed before the main subject- Saturn's main rings, therefore, the comma should be followed by words such as however, but etc. "They are in fact" does not amount to any contradiction/irony but just stating a fact. So the contrast is not clear. Additionally, the meaning of the sentence is debatable. The second clause literally means that saturn's main rings are composed of separate icy ringlets only when you view them up close, i.e. what if we don't view them up close, would this change the composition? The answer is no.
The option C - Saturn's main rings, when viewed from a distance, may appear to be smooth and continuous, though when viewed up close they are in fact composed of thousands of separate icy ringlets.
This choice starts with the main subject- Saturn's main rings followed by an appositive. We can trim the sentence to - Saturn's main rings may appear to be smooth and continuous- Clause 1
Though when viewed up close they are in fact composed of thousands of separate icy ringlets.- Clause 2.
The problem with the sentence is evident. Clause 2 should begin with the main subject noun or pronoun in this case, however, it begins with - though when viewed up close. Therefore, this sentence is not contrasting the subject in a parallel manner. Moreover, reading the clause 2 literally gives a similar meaning error as in choice B
Coming to choice D - When viewed from a distance, Saturn's main rings may appear smooth and continuous, but closer viewing reveals them to be composed of thousands of separate icy ringlets.
The opening phrase- When viewed from a distance correctly modifies Saturn's main rings- We are okay upto this point. Now I read the sentence without the opening phrase and it looks like
Saturn's main rings may appear smooth and continuous, but closer viewing reveals them to be composed of thousands of separate icy ringlets.
The sentence structure - "Saturn's main rings may appear X but closer viewing reveals them to be Y" gives a better contradiction than the first three and the placement of modifier is okay. We may have a slight inhibition regarding the parallel construction so let us check choice E
Choice E- Though composed of thousands of separate icy ringlets if viewed up close, the main rings of Saturn may appear smooth and continuous when they are viewed from a distance.
This sentence has major flaws- The first flaw is a meaning error similar to choice B - read the first part of the sentence literally. Moreover, the subject to be modified should be "Saturn's main rings" and not "the main rings of Saturn" ( GMAT likes subjects in active voice) .
Therefore, D is the best possible choice.