Answer: Option A.
Step-by-step explanation:
Suppose you have a rectangle in the plane X-Y, such that one of the vertices of the rectangle is on the point (0,0) and the rectangle has sides parallel to both x-axis and y-axis.
Now, you rotate it around the y-axis. The created figure will be a cylinder with a radius and height depending on the measures of the original rectangle.
Now, if you cut it horizontally (this means that you have a cut parallel to the x-z plane) is equivalent to have a view of the cylinder from the top (as if you where on the positive y-axis), and this figure will be a circle, so the correct option is A:
