What is the effect of the underlined sentence on page 2?

Underlined sentence: "It's filling up the deepest gulf that separates class from class and soul from soul."

Context: "Playing! The hardest job I ever tackled, make no mistake about that, Mother. But you have no idea how frightfully interesting it is to take a human being and change her into a quite different human being by creating a new speech for her."

Answer options:
It conveys the idea that Higgins thinks his experiment is noble.
It demonstrates Higgins's sense of humor.
It reveals Higgins's modesty about his work.
It shows that Higgins is curious about human nature.