Which two-dimensional shape can be rotated about the y-axis to create a cylinder which has a smaller diameter than height?

The answer is a rectangle such that the height is greater than its width

imagine it yourself
get a normal piece of rectangular paper such that you have one side is greater legnth than other

fold the paper in half hotdog style (fold paralalell to long side)

put a pencil or skewere along that crease you made and tape it there, flatten the paper

then spin the skewer
you will get a cylinder with a height more than width of diameter