IMO D
Since 2005, a 50-acre chunk of hillside — almost eight million cubic yards of rock, enough to fill 60 football stadiums to the top of the bleachers, eight stories up — had been sliding slowly downhill in the one of the canyons outside Malibu; if and when it will stop remains unclear to geologists.
A. rock, enough to fill 60 football stadiums to the top of the bleachers, eight stories up — had been sliding
Wrong Tense had is not needed here
B. rock, which is enough to fill 60 football stadiums to the top of the bleachers and eight stories up — had been sliding
Wrong Tense had is not needed here
C. rock, an amount that is enough to fill 60 football stadiums to the top of the bleachers, which is eight stories up — had been sliding
Wrong Tense had is not needed here
D. rock, enough to fill 60 football stadiums to the top of the bleachers, eight stories up — has been sliding
Showing the present action.
E. rock, an amount that is enough to fill 60 football stadiums to the top of the bleachers, which is eight stories up — slid
Wrong Tense, Slid means the action is complete.