Which sentence contains parallel structure?

a. this peaceful protest the workers staged in the parking lot has been witnessed by drivers, pedestrians, and residents of the neighborhood.
b. the workers staged a peaceful protest in a parking lot of this active community, and it has been witnessed by all the pedestrians who travel here.
c. a peaceful protest, witnessed by many, was held today in this parking lot; everyone who witnessed the workers’ actions will understand their ideals.
d. every driver traveling down this road, every pedestrian crossing this street, every resident of this neighborhood has witnessed the workers’ peaceful protest.