Final answer:
The most serious threat to internal validity in the physical fitness program study for nurses is selection bias, which occurs if the experimental and control groups are not equivalent at the study's start.
Step-by-step explanation:
The most serious threat to internal validity in the study where a nurse researcher implemented a physical fitness program for nurses in one hospital and used nurses in another hospital as the control group would be selection bias.
Selection bias occurs when the participants in the experimental group and control group are not equivalent at the start of the study.
If the nurses in the intervention group happened to be more health-conscious or motivated to exercise prior to the start of the study, the outcomes could be confounded by these pre-existing differences rather than the physical fitness program itself.
To ensure internal validity, random assignment would be ideal, as it minimizes pre-existing differences between groups.
However, as the question suggests, ethical issues can arise with random assignment when it comes to interventions with known beneficial effects, like exercise.