we know that
surface area=2*area of the base+perimeter of base*height
area of the base=b*h/2
b=8 ft
h=15 ft
so
area of triangle=8*15/2-------> 60 ft²
perimeter of base=a+b+c
a=15 ft
b=8 ft
c=?
applying the Pythagoras theorem
c²=a²+b²------> c²=15²+8²------> c²=225+64------> 289
c=√289------> c=17 ft
perimeter=15+8+17=40 ft
height of the prism=20 ft
surface area=2*60+40*20------> 920 ft²
the answer is the option
B. 920 ft 2