Which is a trait of an index fossil? a. resembles a living species b. found in a limited area c. found in a limited time span d. a particular kind of organism

The best answer is C) Found in a limited time span

Index fossils are used to date the soil, nearby fossils, and other features of the area in which they are found. If the species fossil is known to have lived in a very specific time period, then the elements surrounding the fossil can be assumed to also occupy that time period. 
Naturally, the shorter the period in which the species existed, the better it is as index fossil.