Final answer:
The best initial intervention to improve functional mobility in an individual with a stable humeral neck fracture is pendulum exercises, which are gentle and help maintain shoulder mobility without causing additional stress.
Step-by-step explanation:
The question asks about the best initial intervention to improve functional mobility in an individual with a stable humeral neck fracture. Considering the options provided and incorporating principles from physical therapy, the most appropriate initial intervention would likely involve non-resistive and gentle exercises to promote healing and avoid exacerbating the injury. Initially, pendulum exercises would be beneficial as they are gentle and help maintain mobility in the shoulder joint without causing undue stress. These types of exercises typically precede other therapeutic modalities such as resistive exercises and heat application, which are more appropriate further along in the healing process once some healing has occurred.