Answer: Species.
Step-by-step explanation:
A species can be defined as the largest group of organism that consists of two individuals of the opposite sex or the breeding types can produce an offspring which is able to reproduce by the process of sexual reproduction.
Ever year the cluster of hundreds of Danaus plexippus moves to another place in search of favorable conditions.
These organism are known as species and they migrate from one place to another for sometime and then they will return back and they will repeat this activity every year.