Type of prism is decided by the name of base. Like it can be triangular, square, pentagon etc.
Given that prism is triangular so that means it's base is a triangle.
Given that Brad cuts the given triangular prism with a plane parallel to the base of the prism.
Now we have to find the shape of the cross section of that plane.
It is obvious that the plane obtaind after cutting the ptrism parallel to the base will be of same shape as base.
Hence required plane will be a triangle.
