Researchers condition a flatworm to contract its body to a light by repeatedly pairing the light with electric shock. the stage in which the flatworm's contraction response to light is first established and gradually strengthened is called

This is the acquisition stage. During this stage, the learned behavior is being taught and the learner begins to link the stimulus with the response. After a time, the behavior does not need to be linked to the stimulus (the shock) for it to take place and it can automatically occur whenever the light is presented.