Folds in the earth's crust form mostly a. on the inside of volcanoes. b. in the center of tectonic plates. c. along tectonic plate boundaries. d. on the ocean floor.

Folding and faulting are diastrophic process that occurs when the pressure deep within the lithosphere cause the earth's surface to buckle, bend and split apart. 

Folding is a product of the collision between two tectonic plates that causes the crust to rise up in folds during or after an earthquake. 

Thus, the answer is C. It happens along tectonic plate boundaries.