A & C are wrong, "the others" & "others" are all nouns, "other" & "the other" are adjective.
Moreover,the word "but" infers that "the other" is right.
To sort the students:
some ...others...others.
you also can express like this:
some...other students.other students...
you can say like this on if the students are in two groups:
some...the others...!
or some...the other students...!