Question

where do earthquakes happen most frequently?
at faults in the crust
inside folds in the rock
around mountains

Explanation:

Earthquakes occur due to the movement of tectonic plates, which happens at fault lines in the Earth's crust. Faults are areas where rocks break and move. Folds in rocks are related to mountain - building but not the main source of frequent earthquakes, and mountains themselves don't have the concentrated seismic activity as fault - zones in the crust.

Answer:

A. at faults in the crust