Large, slow brain waves called delta waves are most associated with which stage of sleep?

Deep stage 3 of NREM

Explanation:

Delta waves are a type of waves that are recorded in an EEG and has a frequency of 0.5 to 4 Hertz. These waves are known detected during the deep stage 3 of NREM and hence this stage is also known as slow wave sleep. Out of all the brain waves that are detected during an EEG, these waves are the ones with the highest amplitude and the slowest in nature .These waves are detected first at stage 3 and then dominates the stage 4 of the sleep. Since it is in the stage 3 that these waves are detected, the sleep during this stage is also known as delta sleep.