Final answer:
The characteristics of an undecidable problem are best described by the fact that there is no algorithm that can solve all instances of the problem.
Step-by-step explanation:
The statement that best describes the characteristics of an undecidable problem is c) No algorithm can solve it for all cases. An undecidable problem is one for which there is no general algorithmic method to determine the answer for any given input.
For example, the Halting Problem is a well-known undecidable problem where we cannot determine, using a general algorithm, whether an arbitrary computer program will halt or run forever. The nature of undecidable problems is such that they cannot be resolved definitively using a finite series of algorithmic steps, which in turn implies that there is no single unique solution or a way to compute them 'easily' in all instances.