Which of the following types of fitness training is the best choice for an individual who wishes to improve overall muscle definition and tone?
A. Variable-resistance training with heavy weights and a low number of repetitions

B. Static stretching of major muscles and joints in which stretches are held for at least 30 seconds

C. Dynamic stretching involving sports movements in which reach, force, and speed are gradually increased

D. Strength training with a moderate degree of resistance and a high number of repetitions