Which option correctly lists three geologic features that help scientists determine the relative ages of rocks by their positions? 1. Magma, erosion, and intrusions. 2. Erosion, magma, and cross-cutting relationships. 3. Intrusions, index fossils, and magma. 4. Index fossils, intrusions, and cross-cutting relationships.

The correct option that lists three geologic features helping scientists determine the relative ages of rocks by their positions is: 4. Index fossils, intrusions, and cross-cutting relationships.

Explanation:

• Index fossils are fossils of organisms that were widespread but existed for a relatively short period. Their presence in a rock layer can help geologists date the rock relative to other layers.
• Intrusions refer to igneous rock formations that cut across existing rock layers. The principle of cross-cutting relationships states that the rock being intruded upon must be older than the intrusion itself.
• Cross-cutting relationships are geological relationships where one rock formation cuts across another. The rock formation that is being cut must be older than the one cutting across it.

These three features - index fossils, intrusions, and cross-cutting relationships are important tools used by geologists to determine the relative ages of rock layers based on their positions and relationships to each other within the Earth's crust.