Final answer:
Tammy Horton's comment best illustrates the motivational variable of 'challenge' as she is encouraging her students to think deeply and apply their knowledge to solve more difficult problems, thereby enhancing their intrinsic motivation.
Step-by-step explanation:
The variable in the Model for Promoting Student Motivation that Tammy Horton's comment best illustrates is challenge. Tammy's statement suggests that she has prepared problems that require students to go beyond their comfort zones and apply their knowledge in more difficult scenarios. This aligns with educational theories suggesting that providing students with tasks that are challenging, yet achievable, can enhance intrinsic motivation. The motivation to overcome a challenge can lead to deeper engagement with the material and a more personalised understanding of the concepts. Moreover, this idea supports the notion that when students are actively engaged in problem-solving, particularly with thought-provoking and estimable problems, they are more mentally involved and have the opportunity to take personal ownership of their learning.