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

The correct option is who .

To understand why this is the correct answer, let's break down the grammar of the sentence step-by-step:

Step 1: Identify the role of the blank space
The blank space requires a relative pronoun to connect the main clause ("The people were from all sections of society") with the relative clause describing them ("were at the demonstration").

Step 2: Identify the antecedent (the noun being referred to)
The relative pronoun refers back to the noun "The people". Since "people" refers to human beings, we must choose a relative pronoun that is appropriate for persons rather than objects or animals.

Step 3: Analyze the options
- who: Used for people when the pronoun serves as the subject of the relative clause.
- whom: Used for people when the pronoun serves as the object of the relative clause or a preposition.
- which: Used for inanimate objects, concepts, or animals, not human beings.
- whose: A possessive relative pronoun used to indicate ownership or association.

Step 4: Determine the grammatical function in the relative clause
In the relative clause "__________ were at the demonstration", the verb is "were". A verb requires a subject. Therefore, the pronoun must be in the subject case. Since "who" is the subject relative pronoun used for people, it is the correct choice.

Thus, the complete, grammatically correct sentence is: "The people who were at the demonstration were from all sections of society."