Final answer:
Momentum, which takes into account mass and velocity, is the factor associated with the speed and control of an exercise.
Step-by-step explanation:
The safety and effectiveness exercise factor that considers the speed and control of an exercise is momentum (option a). Momentum is related to the mass and velocity of a moving object. In the context of exercise, controlling momentum is crucial for ensuring movements are performed with the correct technique, minimizing the risk of injury, and maximizing the effectiveness of the workout. In contrast, alignment refers to the proper positioning of body parts, stability is about maintaining balance, and biomechanics involves the mechanical laws relating to the movement or structure of living organisms.