Answer & Solution
Identifiers can be of any length as long as they follow the naming rules (e.g., start with a letter or underscore, followed by letters, digits, or underscores).
However, extremely long identifiers are discouraged because they reduce code readability and may not be supported well by some tools or editors.
Option A (79 characters) refers to the recommended maximum line length in PEP 8, not identifier length.
Option B and C (31 and 63 characters) were limits in older languages like C, not Python.
Therefore, the correct answer is Option D: none of the mentioned.