Final answer:
The idiom 'night owl' means that the student's dad likes to be active late at night, which explains why he prefers to work the late shift.
Step-by-step explanation:
The idiom 'night owl' refers to a person who is naturally inclined to stay awake and be active late at night. In the context of the statement 'My dad works the late shift. He says he prefers it because he is a night owl,' the idiom is used to explain that the student's dad likes to work late because it suits his natural tendency to be more alert and active during the night hours. Hence, the correct meaning of the idiom in this context is:
D. Dad likes to be active late at night.