I believe the answer is C) frightened. A participle is a word formed from a verb, and used an an adjective or a noun. Since frightened and climbed are both verbs, that narrows it down to either C or D. From there, frightened is an adjective, while climbed is nor an adjective or a verb.
Example: If you want to explore the effect of salt on a plant growth, the control group would be a set of plants not exposed to salt, while the experimental group would recive the salt treament, so you answer is control group.
