Which type of stress causes rocks to fold?

A bend in a set of rocks caused by compression.

The rock goes back to its original shape when the stress is removed. A bend  or fold in the rocks is caused by compression. Folds are easiest to see in sedimentary or volcanic rocks that were deposited horizontally.

Or in easier words, compression can be defined as the action or state of being squished down or made smaller or more pressed together.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.