Final answer:
Planck introduced the concept of energy quantization to explain the blackbody spectrum, as classical physics failed to accurately predict the observed radiation behavior, particularly avoiding the 'ultraviolet catastrophe'.
Step-by-step explanation:
The key aspect of the blackbody spectrum that forced Max Planck to propose the quantization of energy levels in atoms and molecules was d) The failure of classical physics to explain observed data. Classical theories predicted an 'ultraviolet catastrophe,' where an infinite amount of energy would be emitted at short wavelengths. However, this did not match experimental data, which showed that the radiated energy peaks at a certain wavelength and then declines. Planck proposed that atoms and molecules emit and absorb energy in discrete amounts, or quanta, to resolve this issue. This quantization successfully explained the shape of the blackbody spectrum, and was a significant departure from classical physics.