Concept:Reciprocal pronouns express a shared action among people, and "one another" suits a group of more than two.
Explanation:The sentence refers to "all the students," which means more than two people in a group.
The intended meaning is that every student should help the others in a mutual way.
"One another" is the correct reciprocal pronoun for such a group.
"Each other" would normally describe two people, but it is not among the options.
"One other" and "the other" wrongly suggest a single particular person.
"Themselves" is reflexive, not reciprocal, so it does not fit the meaning.
Therefore, the completed sentence is: The principal advised all the students to be of help to one another.
Answer:Option D: one another