Answer
h = zero.
Work Step by Step
The size of a quantum of light energy is hf. If the discretized energy of light were to instead become continuous, that would be akin to taking the limit as h approached zero. In the same way, the de Broglie wavelength of massive particles, h/p, would also become zero.
In short, in a world with no quantum effects, Planck’s constant h would be zero.
