Nadia used to run a lot but her knees have been bothering her lately she wants to purchase a piece of fitness equipment that will help her keep up her cardio endurance levels while not putting a lot of pressure on her knees and joints what is the best purchase for nothing Nadia to make

For Nadia's needs, the best option among the choices you provided would be an elliptical trainer.

Elliptical trainers offer a low-impact workout that is easier on the knees and joints compared to running or using a treadmill, which can put more stress on those areas. The elliptical allows for a full-body workout and provides a cardiovascular challenge while minimizing the risk of injury.

The adjustable plate-loaded barbell and the nautilus machine are more strength training options, which may not focus on cardio endurance, and they might not offer the low-impact benefits that Nadia is looking for.

In summary, the elliptical trainer would be the most suitable choice for Nadia to maintain her cardio endurance without putting excessive pressure on her knees.