Hey there!
First, let's distinguish what this (index fossil) even means.
This such word called the (index fossil) is a fossil from a animal or even a (plant) which has the detail's of how long it's life span was and all of that. It also illustrates how it may have looked.
So, from looking at these option's listed above, your correct answer would be (The organism lived for brief geologic time period). From the information that I have listed above, this option went quite great.
Your correct answer would be . . .
[tex]\boxed{\boxed{The \ organism \ lived \ for \ brief \ geologic \ time \ period}}[/tex]
Hope this helps.
~Jurgen
