Select the most appropriate option to fill in the blank.

Pilots must be ______ at all times to keep their passengers safe during the flight.

vigilant

The most appropriate option to fill in the blank is vigilant.

The word "vigilant" means "alert and watchful, especially in order to guard against danger or trouble." It is the most accurate word to describe the state of mind that pilots must be in at all times to keep their passengers safe during the flight.

The other options are incorrect.

• Thirsty means "feeling the need to drink." It is not the correct word to use in this sentence because pilots do not need to be thirsty to keep their passengers safe during the flight.
• Distant means "far away or remote." It is not the correct word to use in this sentence because pilots do not need to be distant to keep their passengers safe during the flight.
• Forceful means "having or using great physical or moral strength." It is not the correct word to use in this sentence because pilots do not need to be forceful to keep their passengers safe during the flight.