Answer:
food pellet
Step-by-step explanation:
Unconditioned stimulation is a stimulus that causes an automatic and natural response to a specific situation. In the case of the situation shown in the question, the unconditioned stimulus is the food pellet, because the rats when they smelled vanilla knew they would get a food pellet in the left corner of the cage, so when the rats felt the vanilla smell automatically shifted to the left corner of the cage, even if they didn't receive the food pellet.