Folding of rock layers occurs where pieces of the earth's crust
a.
slide sideways without pushing into each other.
b.
do not move at all.
c.
gradually stretch apart and become thinner.
d.
gradually squeeze together and move upward.

Folding of rock layers occurs where pieces of the Earth's crust gradually squeeze together and move upward

Option D

Explanation:

Rocks become folded when the Earth's outside layer or crust is compacted. Pressure (squashing) happens as structural plates are pushed together and the outside layer ends up shorter and thicker, building mountain extents like the Alps or Himalayas. The stones are collapsed and transformed at profundity.  

Folds result from the moderate distortion of rocks. This happens at the underground where the stones are experiencing the squeeze and temperatures are higher.