Which of the Following Is a Definition

Which of the following is a definition?

A) The reason osculation evolved from being a mathematical term to one referring to kissing is that the images of the forms created through osculation resembled people kissing.
B) The walls of Disney Hall curve upward like sails blowing gently in the wind.
C) To whistle, just pucker up your lips and blow like this.
D) Osculation is the contact between two curves or surfaces at three or more common points.