Answer:
Option D, Fossil in layer 6 are less than 540 million years old
Step-by-step explanation:
In any fossil deposition, the youngest of fossils lies at the topmost layers while all other layers below it are much older than it.
Here, layer 1 lies at the bottom and layer 13 lies at the top. Thus, layer 1 is the oldest and layer 13 is the youngest fossil.
In the question , it is given that index fossil of age 540 million years old are found at the layer 5, therefore all layer lying above it will be less than 540 million years.
Therefore, option D is correct.