Answer
Planck suggested that light itself was quantized.
Work Step by Step
Planck postulated that radiant energy was quantized, i.e., emitted in discrete bundles.
This is discussed on page 584.
(Note: the answer in the back of the 12th edition of this textbook, on page S-28, is incorrect in stating that Planck believed that the energy of vibrating atoms, not light, was quantized. This contradicts not only the textbook statement on page 584, but other sources as well.)
