“Constance Culmington, he reflected to himself, was exactly the sort of woman who would buy an island and surround herself with mystery! Nodding his head in gentle approval of his logic, Mr. Justice Wargrave allowed his head to nod... He slept..." Chapter 1, pg. 3
I would say A however I am not sure
The operation or activity of two or more things at the same time or rate
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.