+1 for A. Because, it seems to bring in all premises.
Stated:
Since 1950's Jets have become quieter, more comfortable, faster and fly higher. FE as much as 70% (Probably stated to say it takes a LOT to negate this benefit).
Travel @ cruising altitude is
especially efficient.
What if the improvements since 1950's were fine but, not actually applicable in Kawai islands case because, the trips are shorter (so passengers don't care about comfortable). Trips are shorter, so, they hardly even fly at cruising speed.
Alternatively, What if Jets can take more passengers (50-100) and turbo planes take 10-15 passengers, so, there is not more than 10 passengers per flight, so, the companies feel it is rather economic to deploy turbo planes instead of using jets almost empty. [ Nonetheless, I did not see any answers related to this]
(A) Because trips within Kawaii are relatively short, planes spend little time at cruising altitude.
This answer seems to state the same point(B) As jets have become more popular, their increased price has erased the financial advantages of their fuel efficiency. => We've not been told of any supply/demand equation or how much is more popular and increased price? are they huge enough to negate 70% fuel efficiency. It is stretching.
(C) Turboprop planes have also enjoyed resurgence in other island nations, especially those with extreme climates.
Out of scope
(D) Many of the independent airlines that once served Kawaii have been purchased by larger multinational airlines.
No way related to the topic
(E) Due to Kawaii's remote location, jet fuel is more expensive there than in most other parts of the world.
That would have been the case always. Kawai would have not gone far away in decades. Also, we've been told 70% fuel efficiency is achieved, so, this actually doesn't resolve the paradox.