Question

why does each element have a unique emission spectrum?
varying electron speeds
unique atomic mass
unique spacing of energy levels
different number of neutrons

Explanation:

When an electron in an atom drops from a higher - energy level to a lower - energy level, it emits a photon. The energy of the photon is equal to the energy difference between the two levels. Each element has a unique set of energy levels, so the energy differences between levels and thus the energies (and wavelengths) of the emitted photons are unique, creating a unique emission spectrum.

Answer:

Unique spacing of energy levels