Answer: Group and heard.
Explanation: a noun is a word, other than a pronoun, used to identify any of a class of people, places, or things, or to name a particular one of these. A collective noun is is a type of noun that identifies groups of people and things as a single unit. From the given options, the collective nouns are the words "group" (that identifies a reunion of people) and "herd" (that identifies a large group of animals).