Vamos a abordar el avión por
a. el viaje
b. la puerta de embarque
c. la inspección de seguridad
d. la tarjeta de embarque i think this is a

The correct answer is not A. Im sorry to say this but the correct answer is B. por la puerta de embarque (which will by by the gate). If you use the A option there would be a grammatical mistake putting the word "por" before something that makes no sense. Im a native spanish speaker and a teacher so believe the better way to say is por la puerta de embarque

1) el chico joven 2) el señor mayor