Final answer:
Marti should focus on increasing the intensity of her workout to create a more challenging session within the same 30-minute block, as she is not ready to increase the weight or change her routine.
Step-by-step explanation:
The principle of overload training that Marti should focus on, given that she wants a more challenging workout in the same 30-minute exercise block without increasing weight or changing routines, is B. Intensity; increasing the effort expended during her workout. This can include strategies like performing the same number of repetitions in less time, or increasing the number of repetitions with the same weight within the same time frame. Adjusting intensity is key to creating a more challenging workout without altering the other variables of frequency, time, and type.