Which of the following phenomena is most directly associated with the development of quantum mechanics due to its challenge to classical physics?

The black‑body radiation problem, where classical physics predicted infinite energy at high frequencies (the ultraviolet catastrophe), forced physicists to rethink energy transfer. Planck’s solution introduced discrete energy packets, or quanta, which contradicted the continuous energy assumptions of classical physics. This breakthrough led to the birth of quantum theory. For example, the observed spectrum of a heated metal rod could only be matched by assuming that light energy came in fixed “chunks” rather than a smooth flow.