Final answer:
From 1979 to 1999, Pluto's elliptical and inclined orbit brought it closer to the Sun than Neptune, making option 'a. 1980' the correct answer.
The correct option is A.
Step-by-step explanation:
The position of Pluto in its elliptical orbit has varied over time, sometimes bringing it closer to the Sun than Neptune. This occurred due to Pluto's inclined and eccentric orbit.
In the second half of the 20th century, and into the 21st century, there were periods during which Pluto was indeed closer to the Sun than Neptune.
One of these periods was from 1979 to 1999, meaning that the correct answer to the student's question would be 'a. 1980' when Pluto's orbit brought it closer to the Sun than Neptune's orbit.
The correct option is A.