The device which measures earthquakes is called the Richter scale.

Answer: Option C

The word measures means to determine the size, amount, or degree of something.

In the given sentence, the Richter scale is used to measure the intensity of earthquakes.

Option C: gauges is the correct synonym because it also means to measure or estimate the magnitude or extent of something.

Option A: prevents means to stop something from happening, which is unrelated.

Option B: weighs refers specifically to measuring weight, not general measurement like earthquake intensity.

Option D: calculates means to compute mathematically, which is not the exact meaning in this context.

Therefore, the best synonym for measures here is gauges.