Earth's plates are constantly moving. What evidence do scientists use to support this claim? a) earthquakes
b) volcanic activity
c) GPS measurements

The movement of crustal rocks has been studied by various scientists and researchers. Most of them have identified earthquake activity as the major cause.

Thus the option A and B are both correct. Primarily option A

Learn more about the earth plates are constantly moving and what evidence scientists use to support this.

A) Earthquakes and B) volcanic activity

Explanation:

Scientists found that the surface of our planet is always in motion. ... The knowledge of the Earth's continual motion is based on the work of scientists who study the movement of the continents. This process is called “plate tectonics.” Earthquakes and volcanic activity are a result of that process.

Both a and b are correct! But if it just wants one I would go with earthquakes first!

Hope this helps ya!