An oblique prism has trapezoidal bases.

which expression represents the volume of the prism?

10x3 cubic units
15x2 cubic units
20x3 cubic units
30x2 cubic units

The correct answer is "30x² cubic units".
The volume of a given figure is given by:
volume=(base area) x (height)
In this case the base area will be given by the area of the trapezoid. Which is:
Area=1/2 x h x (a+b)
height,h=x 
a=x
b=2x
thus;
Area=1/2 ₓ x ₓ (2x+x)
=3/2 x²
Given that the height of the prism is equal to 20 units, so the volume will be:
V=(3/2 x² x 20)
V=30x² cubic units
Thus, the answer is 30x² cubic units.