4. how do geologists determine the age of a fault line

When a fault is formed, relatively young layers of the underlying lithosphere are revealed. Geologists apply the technique of 'radiometric dating' or ‘radiometric decaying' on the samples from these layers to determine the age of the fault line.

Explanation:

Fault lines usually occur due to the spreading of Earth's crust owing to the movement of the tectonic plates. It is "impossible to determine the age" of such fault lines based on their visible characteristics. Thus, the technique of radiometric dating is applied to determine the age of the rocks underlying the uppermost surface of the faults on both sides to accurately guess the "age of the fault".

The answer i think is d