The answer for the exercise is the third option, which is: Hexagon.
The explanation is shown below:
As you can see in the figure attached, the cross section is a polygon of six sides and six angles. Therefore, it has six vertexes. In geometry, this type of polygon is known as "Hexagon".
Let x be the unknown number. The equation is going to be 7/28=25/x, now we cross multiply. 7*x=25*28, 7x=700, divide both sides by 7 and you should get 100, So, x=100 and 7/28=25/100