Find the student who looks sad.

Find a student who looks sad.
(Which one is correct and common?)

How does Junsu look?
He looks sad.
Do you know why?
Yes, I do.

He got 100 points in English.
He got 60 points in math.
He got the best points in

Find the student who looks sad.

Find a student who looks sad.
(Which one is correct and common?)
Both are correct, depending on the context. The first one implies that's there's only one student in the group who looks sad; the second one implies that several look sad, and that one of them should be identified.

How does Junsu look?
He looks sad.
Do you know why?
Yes, I do.

He got 100 points in English.
He got 60 points in math.
He got the best points in
These all look fine except that the last sentence isn't finished!