In twentieth-century music, what began to happen to the difference between consonant and dissonant chords?
a. they began to provide resolution.
b. they began to disappear.
c. they began to provide stability.
d. they began to create tensions that resolved.

b.They began to disappear.                        

Explanation:

              The effects of the consonant chord and the dissonant chords on the ears of the listener was recognized in western music as early as 5th century BC.

            Consonant chords are harmonious sound  and are pleasing to the ears, while dissonant chords are clashing sound  and elicits a feeling of tension in the brains. The consonant and dissonant chords affects the human mind. The consonant sounds gives a happy feeling while the dissonant sound gives a sad feelings to our mind.

           But in the twentieth-century, Arnold Whittall initiates the changing approaches to the harmony and concept of tonality. The twentieth-century music showed the most diverse in the music world. Consonant and dissonant sounds began to disappear. Arnold Schoenberg develop a theoretical model of music in the twentieth-century known as the "twelve-tone music".

Hence the answer is

b.They began to disappear.