C

Step 1: Understand that electron energy levels are quantized states that electrons can occupy in an atom.
Step 2: Recognize that spectral lines result from the transition of electrons between these energy levels, emitting or absorbing light at specific wavelengths.
Step 3: When electrons move from a higher energy level to a lower one, they emit photons, which correspond to the spectral lines observed in atomic spectra.
Step 4: Therefore, the presence of distinct spectral lines in atomic spectra provides direct evidence of discrete electron energy levels.
Hence, the phenomenon that suggests the presence of electron energy levels in atoms is C: Spectral lines.