Answer:
A. Several of the speakers suggested that much remained to be done.
Step-by-step explanation:
An indefinite pronoun is a pronoun (which is a substitution for a noun, noun phrase or "antecedent") that refers to one or more objects, beings, places, ideas, etc. that are not specified. "Several of the speakers" is an indefinite pronoun because it is vague, it does not tell us to whom speakers it is referring to exactly.
Other examples of indefinite pronouns are another, anything, any, anybody, anyone, either, neither, nobody, no, none, someone, some, each, everybody, many, few, and all.