The cross section from slicing the figure vertically would be a rectangle.
When you slice a 3D shape vertically, you're essentially creating a 2D representation of that shape along the plane of the cut.
In the image, the figure is a triangular prism. When sliced vertically, the cut will pass through all three faces of the prism that make up the rectangle.
Therefore, the resulting cross section will be the same shape as the face of the prism that is perpendicular to the cut, which in this case is a rectangle.
So, the answer is B. rectangle.