Plants are more efficient at acquiring carbon than are fungi, in the

You could drive yourself absolutely nuts on this one if you get tunnel-vision, and obsess over the comparison in the underlined portion, as discussed in last Wednesday's verbal chat (transcript is available here). But the action is mostly in the non-underlined portion in this sentence. Two things you should ideally notice:

• "Plants are more efficient at acquiring carbon than are fungi, in the form of carbon dioxide..." --> this is going to make a whole lot more sense if "in the form of carbon dioxide" is right next to "carbon", not "fungi."
• Plants are more efficient at acquiring carbon than are fungi, in the form of carbon dioxide, and converting it to energy-rich sugars. --> looks like some parallelism needs to happen here, so we'll need to keep an eye on that, too

"...fungi, in the form of carbon dioxide..." really doesn't work. Eliminate A.

Same mistake as in (A). Plus, I'd argue that the underlined portion is a little bit confusing, even if we ignore the rest of the sentence: it seems like this could be saying that the plants acquire carbon more efficiently than plants acquire fungi. And that doesn't make sense, obviously. (For more on meaning issues in SC, check out our long-winded Topic of the Week.) Either way, eliminate (B).

Looks good! Keep (C).

This isn't a sentence at all, since the subject "plants" never actually performs an action. Eliminate (D).

Arguably the same meaning issue as in (B). Plus, the parallelism doesn't work: something has to be parallel to "converting", since that's the word that follows "and" in the non-underlined portion... and we really don't have any options at all. Eliminate (E).

(C) is correct.