The group of organisms that has existed for the greatest length of time is : ( 1 ) Gastropods
According to fossil records the group of organisms that have existed for the greatest length of time in the options given below is the gastropods . The gastropods have existed for more than 500 million years ago. They are one of the longest existing group of organsims which consist of snail-like invertebrates.
Hence we can conclude that The group of organisms that has existed for the greatest length of time is Gastropods.
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