The introduction of low-cost airlines => delays increased at nation's busiest airports.

To tackle this, takeoff and landing slots airports must be re-allocated preferably towards traditional airlines, over low-cost airlines.

A) The primary cause of delays at the nation's busiest airports is weather-related. Weather could be a factor determining the departure and arrival of airlines and even if the airport authorities pay significant attention towards traditional airlines. The effective in the solution introduced by airport authorities will have a lag in between the problem and solution. (Wrong)

B) The delays caused by traditional airlines have continually increased, while those caused by low-cost airlines have continually decreased since the introduction of low-cost airlines. If low cost airlines have lower delay records the authorities must put significant attention towards low cost airlines, but since they are paying attention on scheduling of traditional one. This statement is negating the passage. (Wrong)

C. Low-cost airlines account for a higher proportion of the delays than do non-low-cost airlines. The statement is justifying why the authorities are putting more efforts on traditional airlines than low cost airlines. <<<Answer>>>

D Traditional airlines already account for approximately ninety percent of all takeoff and landing slots at the nation's busiest airports, as low-cost airlines tend to fly into and out of less busy airports. No stats mentioned regarding the percentage of airlines used and scheduled (out of scope) (wrong)

E. Low-cost airlines, on average, account for a much lower proportion of the delays than do non-low-cost airlines. Same As B, if low cost airlines were efficient than traditional ones, they would have been put ahead not traditional airlines. (Wrong)

Answer is C