If anything, having the same number of accidents as low-fare airlines would support the idea that major airlines are safer, since they’ve been around longer than low-fare airlines. Also, (A) simply doesn’t point out the argument’s key vulnerability.

The agent says low-fare airline records are too short to be reliable while major airline records are not. His argument is vulnerable because it draws a general conclusion about passengers’ safety on different airlines without actually establishing the airlines’ safety at all.

The agent concludes that major airlines are safer because their safety records are more long-standing and reliable. However, he fails to consider that those reliable records might actually show that major airlines are unsafe.

The agent assumes that airlines with more reliable records are safer. But he doesn't necessarily assume that the safest airlines are the most reliable in documenting their safety.

The agent is simply saying that major airlines are comparatively safer than low-fare airlines. As long as they have fewer accidents than low-fare airlines, it doesn’t matter whether major airlines still have some accidents.