I believe that the best answer for this question is an adjectival prepositional phrase because it describes a noun. It modifies the words "a pygmy hippo," which is the subject of the sentence and therefore a noun. Adjectival prepositional phrases modify nouns, while adverbial prepositional phrases modify verbs, adverbs, or adjectives. Hope this helps.
Notice how the word soliloquy is a bit like the word 'solo'. This is basically what it is: a solo in drama, where somebody is projecting their inner thoughts. This gives the audience direct access to the character's feelings.
