Answer:
Ques 1)
          Circle
Ques 2)
        Square
Ques 3)
           Circle.
Ques 4)
            Rectangle
Ques 5)
            Circle.  
Step-by-step explanation:
We know that when a slicing of a figure i.e. a 3-dimensional figure is done parallel to the base then the two dimensional  figure so obtained is similar to the base of the figure.
Ques 1)
The 2-D cross section that is obtained on slicing a sphere by a horizontal plane is:
                           Circle.
Since on slicing a sphere we will always obtain a circle.
Ques 2)
The cross section that is formed by slicing a cube by a horizontal plane is: 
                                  Square.
As we know that a cube is a three dimensional figure that is formed by six faces all of which are squares.
Hence, the base of a cube is also a square.
Hence, we will obtain a square on slicing cube horizontally.
Ques 3)
If a plane slices a cone parallel to it's base then the cross-section that is obtained is:
                                  Circle.
Since the base of a cone is a circle and when the slicing is done parallel to the base then the cross-section so obtained is similar to the base of original figure.
Ques 4)
When a rectangular pyramid is sliced parallel to it's base by a plane parallel to it's base then the cross-section obtained is:
                                    Rectangle.
Since a rectangular pyramid has 5 faces , 4 of which are triangles and one face is a rectangle which is the base and when the slicing is done parallel to the base of rectangular  pyramid we'll obtain a rectangle.
Ques 5)
A horizontal plane intersects a cylinder parallel to its base, The 2-D cross-section so obtained is:
                        Circle.
Since the base of a cylinder is in the shape of a circle.Hence, circle will be obtained.