C. 409.98 ft³
The formula for the volume of a frustum is ...
... V = (h/3)(B₁ + √(B₁B₂) + B₂)
where B₁ and B₂ are the areas of the bases.
Here, we have ...
... B₁ = (6 ft)(8 ft) = 48 ft²
... B₂ = (14 ft)(18 ft) = 252 ft²
So the volume is ...
... V = (3 ft)/3 × (48 ft² + √(48·252) ft² + 252 ft²) ≈ 409.98 ft³