This is so confusing: which two-dimensional figure could be a cross section of a rectangular pyramid that has been intersected by a plane perpendicular to its base and not through its vertex a a rectangle b a triangle c a hexagon d a trapezoid

The answer is D) a trapezoid.

Take a look at the diagram below.
The red cross section is where the green plane and the black pyramid intersect.
You can see how the plane is perpendiuclar to the base. (They form a right angle.)
If we put the pyramid on a table, the plane would be vertical.
Imagine chopping the pyramid vertically. The part that gets exposed is a trapezoid.
If that chop went through the vertex, (the point at the top) however, the cross-section would end up being a triangle.