The pilot or flight attendant __ usually alert during the flight.

Answer and Explanation:

Even though this question is missing the instructions, we can assume it is a "fill in the blanks".

The sentence is missing a verb. Due to the position of "usually", we can say the verb that is missing either the verb "be" or a modal verb - main verbs would come after "usually", not before. Since there is no other verb in the sentence, we can eliminate modal verbs and choose verb "be" as the best option. Modal verbs need another verb to complement their meaning; the verb "be", in its turn, can work on its own.

The subject is compound, joined by "or". In this case, the verb agrees with the part of the subject that is closest to it. Since "attendant" is singular, the verb will also be singular. Therefore, the correct sentence would be:

- The pilot or flight attendant is usually alert during the flight.

the correct answer would be:  

a. adverb phrase

the