Question

When rocks are put under stress, they may fold, causing ________ to form.

Answer 1

Faults are cracks on the ground that is caused by the movement of the plate tectonics.  They usually cause rocks to break. 

Anitclines and synclines form when rocks are also put under stress, but they fold, not necessarily break apart. They create sharp folds. 

Island arcs are formed through volcanic events and not rocks. 

Mid ocean ridges are formed when oceanic plates move apart.  

Looking at their differences, the answer to your question is B.