Final answer:
The NS (Neutral Stimulus) in this scenario is the mention of 'tomorrow' by Ms. Fenton.
Step-by-step explanation:
In this scenario, the NS (Neutral Stimulus) is the mention of 'tomorrow' by Ms. Fenton. When Ms. Fenton mentions what is planned for class 'tomorrow,' it acts as a neutral stimulus because it does not initially elicit a strong response from the students. However, through repeated pairings with the unconditioned stimulus (class ending) and the subsequent closing of books and packing up of backpacks (conditioned response), the mention of 'tomorrow' becomes a conditioned stimulus that triggers the conditioned response of students closing their books and packing up their backpacks.