Answer:
trapezoid
Step-by-step explanation:
You want the shape created by a vertical plane intersecting a rectangular pyramid.
Lines
When the plane is parallel to one edge of the base, the lines at the top and bottom of the intersection will be parallel to that edge, hence parallel to each other. The top line is shorter than the bottom line.
A 4-sided plane figure with one pair of unequal parallel edges is a trapezoid.
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Additional comment
This is more easily seen when the hidden edges are shown, as in the attachment. Assuming the peak of the pyramid is above the center of the base, the trapezoid will be isosceles.
