The main thing in this sentence is that it does not test idioms or comparisons. It is testing the modifiers...
The number of people flying first class on domestic flights rose sharply in 1990, doubling the increase of the previous year.
A) doubling the increase of
B) doubling that of the increase in
C) double as much as the increase of
D) twice as many as the increase in
E) twice as many as the increase of
the subject of the sentence is 'the number of people' and not the rise of 1990. If the rise was the subject then it would be a comparison sentence and option C would have been the right sentence. ie the rise was double as much as the increase of... The rise and the increase are compared and both are singular and so as many as would be wrong, and as much as would be preferred...
Twice also does not go with the 'rise' and 'increase'. Twice goes with the countable nouns...
Hence even if it was a comparison sentence, options D and E were out...
Hence the confusion would have been between A and C...
This is strictly a modifier question where in the subject has risen to something something...
Hope this helps...
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