What shape would be the cross section from a vertical slice of a hexagonal pyramid?

It depends

Step-by-step explanation:

The shape will have a number of sides equal to the number of faces it intersects (counting the base). Depending on the shape of the hexagonal base, the symmetry of the peak, and where the slicing is done, the vertical slice could have 3, 4, 5, 6, or 7 sides, so could be anything from a triangle to a heptagon.