The people __________ were at the demonstration were from all sections of society.

The correct option is who.

Explanation:

In English grammar, relative pronouns are used to connect a clause or phrase to a noun or pronoun.

1. "Who" is used as a subject relative pronoun referring to people. In the sentence, "The people __________ were at the demonstration", the relative pronoun acts as the subject of the verb "were". Therefore, who is the correct choice.

2. "Whose" indicates possession (e.g., "the person whose car was stolen").

3. "Which" is used for animals, objects, or ideas, not for people.

4. "Whom" is an objective pronoun used when the person is the object of a verb or preposition (e.g., "the person whom I saw").

Thus, completing the sentence gives: "The people who were at the demonstration were from all sections of society."