In following example, which element is the subdominant element? a. black border around composition b. gray and white striped background c. green background d. white circle be the right answer

The green background is the subdominant element

In art terminology, dominance is anything that attracts the viewer's eye in the first sight. If we look at  the picture for the first time the eyes attract the circle and white image at the very first sight so it is the dominance object but the green background behind it is not so much attractive for the first time that is why it is called subdominant element.