Question

the diagram represents the rock layers and fossils found in four separate rock outcrops.
question 1
which fossil is the best index fossil?
outcrop 1
outcrop 2
outcrop 3
outcrop 4
options:

• it is found in only one of the four outcrops.
• it is found in one layer in all four outcrops
• it is found in both the oldest and youngest layers.
• it is found in all four layers in one outcrop.

Explanation:

An index fossil is widespread (found in many outcrops) and existed for a short time (found in only one layer across outcrops). Let's analyze each option:

• The first fossil (stick - like): Found in only Outcrop 4, not widespread.
• The second fossil (eye - like): Check each outcrop. Outcrop 1: has it; Outcrop 2: has it; Outcrop 3: has it; Outcrop 4: has it. Wait, no—wait, looking at layers. Wait, no, index fossils should be in multiple outcrops but limited layers. Wait, re - evaluating:

Wait, the third option (eye - shaped) – Wait, no, let's check the options' descriptions. The option "It is found in one layer in all four outcrops" – an index fossil is useful for correlation, so it should be present in the same layer (time - equivalent) across multiple outcrops. Let's check the fossils:
Looking at the eye - shaped fossil: In Outcrop 1 (middle layer), Outcrop 2 (middle layer), Outcrop 3 (middle layer), Outcrop 4 (middle layer)? Wait, no, the option "It is found in one layer in all four outcrops" matches the index fossil criteria (widespread, same layer across outcrops, short - lived so only one layer). The other options:

• "It is found in only one of the four outcrops" – not widespread.
• "It is found in both the oldest and youngest layers" – long - lived, not good for index.
• "It is found in all four layers in one outcrop" – long - lived in one outcrop, not good.

So the fossil with "It is found in one layer in all four outcrops" is the index fossil.

Answer:

The option with the description "It is found in one layer in all four outcrops" (the eye - shaped fossil's option)