The correct answer is C: Spurious signals at discrete frequencies. The major spectral impurity components of direct digital synthesizers (DDS) are spurious signals at discrete frequencies (spurs). These are unwanted frequency components that appear in the DDS output spectrum. DDS spurs are caused by quantization errors, phase truncation (when the phase accumulator has more bits than the lookup table address), and DAC nonlinearities. These create discrete spurious frequencies in the output spectrum. Spurs are a major concern in DDS design because they can interfere with desired signals or cause problems in receivers. Designers use techniques like dithering, larger lookup tables, and better DACs to reduce spurs. Unlike broadband noise, spurs appear at specific frequencies and can be particularly problematic.