Final answer:
A decline push-up is best modified using a decline bench to adjust the intensity by altering the height of the decline. Medicine balls, chest flies, or classic push-ups are either unrelated or serve as regressions and do not modify the decline push-up itself.
Step-by-step explanation:
A decline push-up can be modified by using a decline bench (Option C). In a decline push-up, the feet are elevated above the level of the head, increasing the resistance and intensity of the exercise as the upper chest and shoulders bear more of the body's weight. To reduce the difficulty or to accommodate different fitness levels, the height of the decline can be altered or different variations of standard push-ups can be performed instead.
Using a medicine ball (Option B) or performing a chest fly (Option D) would not directly modify a decline push-up, as these involve different exercises and muscle groups. A classic push-up (Option A) is a different exercise variation but does not specifically modify the decline push-up; it serves as a regression, meaning it's an easier alternative.