Okay, so here's the sentence according to D:
Meticulously embellishing the elaborate calligraphy, medieval monks, through their manuscript illuminations, communicated their interpretive understanding of the texts they illustrated.
Is there a rule that we can't have two adverbial modifiers in a row (separated or not)? No, but it often causes trouble when we try! Sometimes it makes the intended meaning unclear, and sometimes it just keeps us waiting too long to get to the main clause. In this case, we have an additional adverbial modifier inserted between the subject (monks) and the verb (communicated). This isn't
always wrong, but it's an advanced move that one should typically avoid, and it's generally going to work out badly on SC.
Normally, we'd fix this by placing the modifier before or after the action. In this case, since the clause ends in a long modifier, placing "through their manuscript illuminations" before the clause makes the most sense. The problem is that we have another modifier there. If we start with "Meticulously embellishing . . . , through their manuscript illuminations," by the time the reader gets through all that they are just dying to know the subject! For a similar "traffic jam," look at answer choice E of the Stella Adler problem. (E is also wrong for other modifier-related reasons.)
https://gmatclub.com/forum/as-an-actres ... 20808.htmlSo what do we do? Typically, the GMAT will solve this problem by converting one of the modifiers to something else. The right answer of the Stella Adler problem takes one of those adverbial modifiers and puts it into the main clause. The monks problem takes what was an adverbial modifier in A/B/D and turns it into a noun modifier that builds on the initial adverbial modifier. This is not the most elegant solution (C is still fairly bad writing), but it helps to solve the problem.
Another problem with D is that it opens with "Meticulously embellishing
the elaborate calligraphy." It makes it seem as if there is some existing calligraphy that we're already supposed to know about. To avoid this, we can ditch the article ("the"), as C does, or we can add something after "calligraphy" (e.g. "Meticulously embellishing the elaborate calligraphy in their manuscripts, medieval monks . . . ").