Final answer:
The characteristics of an undecidable problem are best described by the inability to be solved using any algorithm, such as the halting problem. Algorithms provide a set path to a solution, whereas heuristics offer practical, though not always optimal, problem-solving shortcuts.
Step-by-step explanation:
The statement that best describes the characteristics of an undecidable problem is b) It cannot be solved by any algorithm. Undecidable problems are those for which no algorithm can be constructed that always leads to a correct yes-or-no answer. An example of such a problem is the halting problem, which asks whether a given computer program will eventually halt (stop executing) or will run indefinitely on a given input.
In contrast, an algorithm is a set of well-defined instructions for carrying out a task, which will eventually produce a result or correctly report that there is no solution. A heuristic is a practical approach to problem-solving that employs a mental shortcut to generate a solution that is good enough under the given circumstances but is not guaranteed to be optimal or even correct.