Answer :
D. When the trees dropped their leaves in autumn.
"When the trees dropped their leaves in autumn, the grounds of the college campus were blanketed with a layer of red and gold leaves."
"When the trees dropped their leaves in autumn, the grounds of the college campus were blanketed with a layer of red and gold leaves."
“last autumn________, the grounds of the college campus were blanketed with a layer of red and gold leaves.”