The best purchase for Nadia, considering her knee issues and desire to maintain cardio endurance, would be:
D. An elliptical trainer
Elliptical trainers are designed to provide a low-impact workout, which reduces stress on the knees and joints compared to running on a treadmill or doing weight-bearing exercises. This will allow her to keep up her cardio endurance while minimizing discomfort. Other options, such as a treadmill, may exacerbate her knee problems due to their higher impact nature.