Identify the 2-dimensional shape created from a diagonal cross section of a cone

An ellipse

Step-by-step explanation:

Take a cone and make a diagonal cut through it, removing a very thin slice. The result is an ellipse.

a / /

/ /

/ / b

/ /

Make a straight line from a to b, it will make an elliptical shape. If we cut perfectly horizontally, we would get a circle, when we angle the cut of our cross section, the shape gets more and more elliptical depending on the angle of the cut