The main purpose of this sentence in the paragraph is to introduce a list of features. Thus 3rd option is correct.
Vampire Squid is a deep-sea monsters which is the smallest of the other creatures. It has the lengths of only six inches. It is most ancient of the deep-sea monsters and according to the scientist one of the last of its kinds.
Vampire squid has the similar features of the other sea creatures such as octopus, fish ,squid etc. First, it has large fins at the top of its head that look like ears. Thus this line is the introduction of the list of features.
Thus 3rd option is correct.
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