Stories like this one usually follow a trigger, a response, a discovery, and a conclusion, so it helps to sort the four sentences into those four roles first.
The trigger has to be the phone call to the airport, since every other sentence in the paragraph only makes sense as something that happens because of that call. The response to a warning call about a hijack would naturally be a search, which matches the frisking of passengers and one being asked to step aside. The discovery comes next: searching that one passenger's jacket pockets turns up "what had been feared," meaning whatever the caller had warned about. The conclusion wraps up the story by crediting the person who made the call as the "responsible citizen" who helped foil the hijack, since "helped to foil" is a summary statement that only makes sense after the plan has actually been stopped.
Sorting the sentences into trigger, response, discovery, and conclusion gives the call, the frisking, the pocket discovery, and the credit to the citizen, in that order.
So, the correct answer is cdba.
A different way to solve this is to follow only the pronouns and referring phrases from one sentence to the next, setting aside the full story for a moment.
Chaining only the pronouns "his," "one of them," and "the responsible citizen" back to their sources confirms the same order.
So, the correct answer is cdba.