Answer:
a rectangle
Step-by-step explanation:
As we can see that there is a cross-section which we called as a rectangle.
Plus there is a triangle which is to be formed but this would be possible when the plane is perpendicular to the base of the rectangle and at the same time it is parallel to the base of the triangular
Now if a plane cuts a figure i.e parallel to the mentioned base therefore the cross section would be received as a figure which would be parallel to its base
