doubling vs twice

The best answer from the choices given is A.
"twice as many as the increase in/of" is incorrect construction.

Here doubling is modifying the entire clause : The number of people flying first class on domestic .........

As far as "twice" usage is concerned, please refer manhattan SC.

A example from MGMAT SC,

The concert was attended by 300 people, twice the previous attendence. - "twice ........" modifies 300 people.
2 X previous attendance = 300 people
usage of twice after "comma" similar to D and E


I hope this helps

Here 2 things need to be remembered.
1. Twice is an adverb and can't be used with 'by'.
2. Double can act as a noun, a verb, an adjective..