Final answer:
The signal processing technique that adds dirt and grit to a guitar sound is called distortion. It creates a more aggressive and edgy tone by clipping the audio signal and adding harmonics.
Step-by-step explanation:
The signal processing technique that adds dirt and grit to a guitar sound is called distortion. This effect alters the sound of the electric guitar, or any other instrument, by clipping the audio signal, which adds harmonics and, as a result, creates a grittier, 'dirtier' sound. This is commonly used in rock and other genres of music to achieve a more aggressive and edgy tone.
The options provided, such as reverb, delay, and chorus, serve different purposes. Reverb adds echoes to replicate the feel of different spaces, delay repeats the sound after a short period, and chorus thickens the sound by duplicating it slightly out of time and tune. None of these effects adds the same type of grit or aggressiveness to the sound as distortion does.