Answer:
The shape of the cross section of the figure that is perpendicular to the triangular bases and passes through a vertex of the triangular bases is:
A rectangle.
Step-by-step explanation:
From the figure attached to the answer we may clearly observe that the cross-section so obtained is a rectangle.
Also a triangle would have formed if the plane is perpendicular to the rectangular base and parallel to the triangular base.
Also, if a plane cuts a figure parallel to the given base so the cross-section so obtained is a figure parallel to it's base.