First look at the picture.
[tex]Surface\ area\ A_S=2\cdot A_1+A_2+A_3+A_4+A_5+A_6+A_7+2\cdot A_8\\\\A_1=15\cdot9=135\ (ft^2)\\A_2=9\cdot4=36\ (ft^2)\\A_3=15\cdot4=60\ (ft^2)\\A_4=15\cdot4=60\ (ft^2)\\A_5=4\cdot6=24\ (ft^2)\\A_6=4\cdot6=24\ (ft^2)\\A_7=4\cdot9=36\ (ft^2)\\A_8=6\cdot6=36\ (ft^2)\\\\A_S=2\cdot135+36+60+60+24+24+36+2\cdot36=510\ (ft^2)\leftarrow\boxed{\boxed{C}}[/tex]
