Answer:
triangle. 
Explanation:
given: a square pyramid
to find : shape of cross section made when a plane perpendicular 
 to base and passes through top vertex. 
 
when a plane cuts the square pyramid perpendicular to base and passes through top vertex we get a cross section made by its two equal slant height and side of the square.
therefore, we get a triangle or more specifically isosceles triangle.
picture is attached.