In the following diagram, four different index fossils are labeled: which of the labeled fossils comes from the youngest organism? a b c d

If you look at the leftmost part of the graphic, it includes a time axis, where the top represents the most recent time and the bottom the earlier times, so just using this axis, we can see that the fossil A is the earliest, or the youngest, because it it the highest one. - This is the correct answer, fossil A. 

Additionally, we could look at the periods - our fossil A is from the Jurassic period. we can compare it to the fossil B, which is from Permian period. Indeed, Jurassic is a younger age than Permian (or  Devonian and Cambrian, the ages of C and D) - so this confirms that A is the best answer.