Dumbbells are most accurately described as:

Dumbbells are most accurately described as a weight consisting of two discs attached to a short bar, which is option C. This design allows for a compact means of resistance training that can be used for a variety of exercises targeting different muscle groups. The short bar connects the weights on either end, facilitating easy handling and maneuverability, which is essential for strength training routines.

In contrast, neck weights are specifically designed for rehabilitation and therapy, making option A less applicable to the broader definition of dumbbells. Weighted plates attached to a barbell, as described in option B, refers to a different type of equipment used in a different manner, typically involving long bars for exercises that require a stable grip with both hands. Finally, option D describes a machine specifically designed for leg exercises, which does not align with the portable and versatile nature of dumbbells. Thus, the most fitting description of dumbbells' structure and use is captured in option C.