Went with E on this. Here's how I got to the answer:
Sundog Airlines in 2015, replaced Turboprop (TP) planes with Regional Jets (RJ). In 2015, their fares (number of bookings) increased by 10-30% as compared to any of the last 5 years. However, they also saw lower profits every quarter of 2015 than any of the corresponding quarters in the last 5 years. CEO thinks that this is because of the increased operation costs of the RJs (Conclusion).
Now to breakdown, we need to take into account a simple concept that profits increase if revenue increases or costs decrease or both. Similarly it decreases if revenue decreases or costs increase or both. Note that the passage just tells us that the number of bookings increased. That might not necessarily always translate to an increase in revenue. Eg. if I sold 100 tickets at $50 each, I make $5000 in revenue. But if I increase the number of bookings to 130 but drop the price to $20, the revenue still decreases to $2600. Thus, we need to understand that increase in fares does not mean increase in revenue. In the conclusion, the CEO does admit that the operating costs have increased due to moving from TP to RJ. And he thinks this increase is causing the profits to go down. However, we just established that the profits could also go down if revenues decrease/they remain the same but the cost increases. Thus, in order to weaken the conclusion, we need to find an option that helps us identify if the revenue is decreasing/flatlining and hence the profits are coming down (and not because the costs have increased)
Option A: Gives us information about other airlines. These other airlines could be charging exorbitant amount of prices to increase profit. It does not tell us anything about profits, revenues and costs of Sundog Airlines. Hence, we can eliminate this.
Option B: This could help us explain why more passengers might be booking Sundog Airlines. But like we established, more bookings does not mean more revenue. Hence, we can eliminate this as this does not tell us anything about decreasing revenue.
Option C: If they are doing lesser number of flights than last 5 years, this should actually reduce their operating costs (not entirely, but still). So if anything, this should help us positively impact profits. Hence, we can eliminate this.
Option D: Again less fuel used per person should overall reduce the operating cost and hence positiively impact profits. However, we are given that operating costs are increasing. Thus, since this does not tell us anything about revenue, we can eliminate this.
Option E: Yes! This exactly tells us how the profits could be going down. Since bookings have increased, Sundog will need to service more customers (which should increase the cost). Thus, for profits to increase, Sundog should also get more revenue from these increased bookings. However, since the new bookings are coming at the same amount that is the actual cost of servicing those bookings, the ratio of revenue/cost is actually decreasing. THis is an alternative explanation to the one in the conclusion. Thus, we can choose this.