780, or C, is correct. Since 9.75 is 1/80th of the actual measurement, we can set up an equation like this:
9.75 = 1/80x
Then multiply each side by 80:
780 = x
The actual building's height is 780 ft.
We can check this answer by plugging in 780 for x in the original equation:
9.75 = 1/80(780)
9.75 = 9.75
Check! ✓