Answer: Second option.
Explanation:
For this exercise it is important to remember that, by definition, a "Cross section" is the two-dimensional shape obtained by the intersection of a three-dimensional body with a plane.
In this case you can observe in the figure given in the exercise, that the the cylinder (which is the three dimensional body) is sliced perpendicular to its base.
By definition , when a plane intersects a cylinder perpendicular to its base, the resulting shape obtained is a Rectangle.
A Rectangle is defined as a quadrilateral whose opposite sides are parallel and have equal lengt. And each angle measures 90 degrees (Angles that measure 90 degrees are also called "Right angles").
To see this better, you can see the picture attached.