PIMSO 2023 - Science - Senior Secondary - National Round Question 17

Question 17

Several phenomena/observations in the 19th to early-20th century could not be explained by classical Newtonian physics, demanding new physical theories which eventually led to the development of quantum mechanics. The following lists these phenomena EXCEPT ONE. Which of the following is NOT a phenomenon that contributed to the development of quantum mechanics?

Discrete colors of line spectra of elemental gasses
The photoelectric effect
Ultraviolet catastrophe in blackbody radiation
Particle-like properties of electrons
Stable orbits of electrons